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A quick guide to floppy disks

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    A quick guide to floppy disks

    It's funny. I hear lots of new/younger users whine and complain how "unreliable" floppy disks and drives are. They aren't. You're just doing it wrong. Cared for properly, floppy disks can be a very reliable medium. Sure, they're not as robust as a modern USB flash drive, but remember: this was the media around which this hardware was designed. They used to work quite well. Occasional problems were had, but by and large, they were quite robust. This hasn't changed.

    First up is determining what kind of drive your computer has. In the realm of IBM PC clones, there are two disk sizes and four common formats (and a couple really uncommon ones).


    5 1/4" drives:

    5 1/4" drives exist in two basic types - double density and high density. There are also two common form factors - half height and full height.

    Full height drives were used on the original IBM PC, and several other machines. They are about three inches tall - twice as high as a modern CDROM drive. These full height drives are always double density. Very early IBM PC's shipped with single sided drives. You're not likely to find single sided disk drives in PC's though - these only appeared on the early runs of the original IBM PC. As far as I know, no clone maker used them. If you have a full height drive in an IBM PC, it's easy to check if it's single or double sided - just look at the heads. In a single sided drive, the upper head is replaced with a felt pressure pad.

    Half height drives were introduced by clone makers, and IBM eventually started using them by the time the AT came out. These are the same height and form factor as a modern CDROM drive. This is where it gets tricky - half height drives exist in both double and high density. On a *genuine* IBM supplied drive, there will be a recessed asterisk in the faceplate if it's double density. You will probably only likely encounter one on an AT - because they only started doing this after high density drives existed. This is not at all a reliable way to tell, however, since any clone drive won't have this marking, and drives made prior to the introduction of high density drives won't have it. Best way to tell is to probably look up the drive's model number, or simply try to read a HD disk. Also, high density disk controllers are very rare in XT's and clones - they don't become commonplace until 286 based machines.

    Here's the difference:

    Double Sided/Double Density (DS/DD): 40 track, 9 sectors per track, 512byte sectors. 360k.
    48TPI, 300 oersted media. 250Kb/sec data rate. 300RPM
    Double Sided/High Density (DS/HD): 80 track, 15 sectors per track, 512byte sectors. 1.2mb.
    96TPI, 600 oersted media. 500Kb/sec data rate. 360RPM

    So, the high density drive has double the number of tracks, and a greater number of sectors per track. The rotation speed is also higher, and thus the data rate is higher. Therefore, there are a lot of incompatabilities here - the higher data rate means a high density drive can't be used on a double density controller. The higher data rate requires different magnetic properties to the media, and thus a different magnetic write current (the oersted rating of the media). The greater number of tracks means the head is narrower.

    Now, high density drives are backwards compatible - you can read DD disks in a HD drive. But writing to them becomes tricky. Because the head on a HD drive is half the width of the DD drive, it'll write and erase only half the track - leaving bits of the old data behind. This means that if you take a disk written in an 40 track drive, and modify or write files to it in an 80 track drive, you may wind up with a disk that reads in the 80 track drive, but won't read in the 40 track drive. There are ways to make it work, I'll explain in a minute.

    Practical upshot: you need high density media for high density drives, and double density media for double density drives.

    How do you tell the media apart though? This is where most people get tripped up. 3 1/2" disks have a little notch to tell the drive if it's a high or double density disk. 5 1/4" disks have no such notch. There is NOTHING to tell the drive what kind of disk you have inserted, beyond the existing information on the disk. DOS can detect what mode to put the drive in, so if you put an already formatted DD disk in a HD drive, it'll just work - reading as it should, and allowing you to write to it (even though it might corrupt the disk, see above). But an unformatted disk (or one formatted for another type of computer) won't tip DOS off to the media format. Put an unformatted DD disk into a HD drive, type format... it'll happily try to format it to high density, and fail on every track. So, if you must use DD media in a HD drive, you need to tell FORMAT what kind of media you have. FORMAT /T:360 in newer versions of DOS, FORMAT /4 for older ones.

    But more importantly, how can you, the user tell the disks apart? You have to look carefully. Disks with manufacturer labels will actually say DS/DD or DS/HD. Another thing to look for is the hub ring. A vast majority of DS/DD media has a reinforcing hub ring, but most HD media does not. Also, the media itself looks different. The disk surface on DD disks is duller, and browner - but HD media is shinier and looks more black. Once you get used to the way the disks look, you won't even think twice about it. But compare some known labeled disks and you'll understand what I'm talking about.

    Because the media is so different, formatting a HD disk in a DD drive will always fail, and trying to format a DD disk as HD in a HD drive will always fail. If, by some weird miracle, it works, it won't be reliable. Also, attempting to format a disk as the wrong density can even damage it - applying too powerful a magnetic field for the correct drive to erase.

    Now, what if you want to write a disk (say, on a newer PC with HD drive) and read or boot it in an XT with a DD drive? You can do it, but it might take a bit of fiddling. If you have new, unformatted media, or a bulk disk eraser, and thus are starting with a clean disk - simply format it as DD in the HD drive, write your data, and move it over. When you have it on the system with the DD drive, COPY the disk onto a blank disk formatted in the DD drive. By ensuring that your "transfer" disk has only been written with the narrow head of the HD drive, and the spaces around the data are untouched, the disk should read every time in the DD drive.

    Now, what about if you don't have virgin media? If you format the disk in the DD drive, then write your data in the HD drive and move it back to the DD drive, you also stand a fair chance of it working. The "formatted" surface isn't as clean as a new surface, but by formatting it in the DD drive and thus erasing data with the wider head, you give less to interfere with the new narrow data recording.

    It's when you start mixing data written with the wide and narrow head that you run into problems. If you format a disk in the DD drive and write a bunch of files to it, then move it over to the HD drive, delete some files and write new ones, then bring it back over to the DD drive... the chances of it working are much lower. Try to avoid doing this.

    Write protect on 5 1/4" disks: The notch in the side of the disk is the write protect mechanism. If the notch is left open, the disk can be written to. Tape over it with a label or tape, and the disk is write protected. Usually, there is a little switch that feels for this hole, but some drives have an optical sensor, so use something opaque.

    3 1/2" drives:

    3 1/2" disks didn't start to become common on PC's until the 386 era. IBM used them in their PS/2 line, but by and large, the XT and AT computers and clones used 5 1/4" floppies. Portable machines like the Toshiba clamshell luggables also used 3 1/2" disks.

    Again, as with 5 1/4" disks, 3 1/2" drives exist in both double and high density. Likewise, there are double density and high density disks.

    Here's the difference:

    Double Sided/Double Density (DS/DD): 80 track, 9 sectors per track, 512byte sectors. 720k.
    135TPI, 665 oersted media. 250Kb/sec data rate. 300RPM
    Double Sided/High Density (DS/HD): 80 track, 18 sectors per track, 512byte sectors. 1.4mb.
    135TPI, 720 oersted media. 500Kb/sec data rate. 300RPM

    Again, we have a higher data rate for high density. But the number of tracks is the same! This means that a high density 3 1/2" drive can write DD disks exactly as well as a DD drive can. No more incompatibility problems between drives.

    Also note that the magnetic properties in the media is *very* similar. So similar, in fact, that back when HD disks were expensive, we used to drill holes in DD disks and reformat them as HD. It wasn't a good idea, it wasn't supposed to be reliable... but it worked pretty well and it saved money, so a lot of people did it. The annoying thing is that it only works well in that direction. Taping over the density select hole in modern (common) HD disks and trying to format it as DD doesn't work as well, if at all.

    So, again, you really need the right disks for the drive, but cheating is possible, it can work in a pinch.

    As mentioned, you can tell the difference between the disks by looking at them. Holding a disk, hub down, with the shutter towards you, there is a hole in the upper right - that's the write protect tab, slide it so that the window is open to write protect the disk, closed to allow writing. The hole in the upper left is the density select hole. If there is no hole, it's a DD disk, if there is a hole, it's a HD disk. HD disks also almost always have the HD logo embossed into them.

    In any event, mucking with this hole (taping over it or drilling it) tricks the drive into thinking the disk is another density. This is not a good idea, due to the unreliable nature of using the wrong media, but it *may* work in a pinch. Some early HD drives (as used in the IBM PS/2) did not check for this hole at all, requiring the user to manually tell FORMAT what kind of disk was being used, just like the old 5 1/4" disks.

    Again, you want to determine what kind of drive your system has. The data rate on a DD 3 1/2" drive is the same as the old DD 5 1/4" disks, the computer simply needs to know that the drive has 80 tracks instead of 40. Thus, some people upgraded XT's with 3 1/2" disk drives. Several clones of the era shipped with DD 3 1/2" drives. Some portable machines also used them. It's easy enough to tell, you can usually just look at the sensors. DD drives will have two switches - one for write protect, and one opposite, in the density select position, for "Disk IN" - to allow the computer to know when a disk was inserted. Not that PC's ever actually used that functionality, but still. HD drives will have three switches - write protect, disk in, and density select.

    By the 386 era, pretty much any PC that had a 3 1/2" floppy drive had a HD drive.

    Fortunately, since the drives autodetect media, and modern (even the USB floppy drives) work with DD disks, 3 1/2" drives are very easy to work with. Since HD drives have no trouble with DD disks, there is no disk interchange issues. DD disks work everywhere, HD disks work only in HD drives.


    -Ian

    #2
    I remember the battle between the 3.5" drive vs the 3" drive. The 3.5" won out. Wish I had one of those 3" drives!

    http://csdl.computer.org/plugins/dl/...2/04070788.pdf

    Also, IBM tried to push the ED 2.88MB 3.5" drives in 87/88 but didn't get much traction.

    I still have an LS 120 3.5" drive that get 120MB per 3.5" disk.

    Comment


      #3
      Originally posted by aoresteen View Post
      I remember the battle between the 3.5" drive vs the 3" drive. The 3.5" won out. Wish I had one of those 3" drives!
      I have some. They were used in Amstrad PCW word processor/computer things. They really weren't great, 3 1/2" was a better format, and fortunately the market saw that. The 3" disks sure seemed cool though! The 3" format was designed as a drop-in replacement for existing DS/DD 5 1/4" drives. It offered no more storage, just cute little cartridges. Amdek marketed them as the AmDisk, and even made drives that worked on Apple II systems, as drop in replacements for the Disk II. Again, quite cool looking, but beyond a neat-o little cartridge, didn't buy you anything in terms of storage. Only now nobody else can read your disks. Oh, and they're more expensive too. The 3 1/2" format actually offered greater storage capacity.

      Also, IBM tried to push the ED 2.88MB 3.5" drives in 87/88 but didn't get much traction.
      Yeah, that lasted about fifteen minutes in the marketplace. NeXT used them for a little while too. They worked OK, but they weren't as reliable as 1.44mb disks, and since IBM pretty much kept it to themselves, it didn't gain any ground because it wasn't a viable interchange format. (i.e. nobody had it except people that bought IBM PS/2's with the option). Later Dell servers (PII Xeon era) snuck in 2.88 drives, but they didn't draw attention to them. I'm not sure why though - perhaps they expected to be able to distribute BIOS updates on them? I dunno.

      I still have an LS 120 3.5" drive that get 120MB per 3.5" disk.
      I don't know if the LS120 quite counts as a floppy disk - it's the same form factor disk, but it's the same kind of servo positioned "removable disk" that the Zip disk was, only in another form factor. Also, the drives are intelligent, and communicate to the computer over the standard hard disk type interface (IDE or SCSI). Sure, the media is floppy, but so is the Zip disk and Bernoulli disk. Also in this category would be the "Floptical" disk - 21mb per disk, and backwards compatible with regular floppies. In any event, none of these media formats really match the PC/XT era hardware

      -Ian

      Comment


        #4
        FWIW, I vote that this be topic be sticky'd, and/or added to the Wiki. Might save some very redundant questions that I've seen appear over and over in even my short time here!
        ---
        Currently seeking:
        * Roland MPU-401/AT (with daughter card header)
        * Magitronic K-156 Keyboard (5pin DIN w/ XT-AT switch)
        I also collect PC and C64 Sierra On-Line software!

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          #5
          I second that vote. It's going to save a lot of questiones from the "just-started-collecting-old-computers" people.
          ~Ian~

          Remember, wherever you go, there you are.

          Comment


            #6
            Originally posted by RetroHacker_ View Post
            I don't know if the LS120 quite counts as a floppy disk
            I use it as my main 3.5" floppy drive in both my modern C2Q Win7 rig, and my XP 3.5/5.25 equipped rig, it will read and write 720k and 1.44mb floppies flawlessly, I would sure call it a floppy
            My Vintage computer/blog site
            Searching for a keyboard for a WYSEpc WY-1100.

            Comment


              #7
              What about 3" disks? I have drives and media for those as well. And then there was the 2" drive used on the Zenith portables.

              There were other MS-DOS formats; chiefest is that used on the NEC PC98 systems. Whether or not you want to call that "compatible" is a matter of debate, but there are versions of Windows that run on it, so it got Microsoft's blessing.

              And at least one or two systems used 720K "quad density" 5" (DSQD, 80 cylinder).

              It might also benefit the discussion if the term "track" was taken to mean "a annular ring of recording surface on one side of a disk" and the term "cylinder" used. The reason is that you'll sometimes see a 48 tpi 5" double-sided drive called by its manufacturer "80 track"; that is, the drive records 40 cylinders, two sides. This would also bring the terminology into agreement with hard disk terminology.

              And, believe it or not, Microsoft, for a time, also defined an 8" MS-DOS format.
              Reach me: vcfblackhole _at_ protonmail dot com.

              Comment


                #8
                Originally posted by Chuck(G) View Post
                And, believe it or not, Microsoft, for a time, also defined an 8" MS-DOS format.
                Supersized DOS files I would love to find an 8 inch disk.
                ~Ian~

                Remember, wherever you go, there you are.

                Comment


                  #9
                  Originally posted by Chuck(G) View Post
                  What about 3" disks? I have drives and media for those as well.
                  The Dysan ones, right? I think you're the only one that has those...
                  And then there was the 2" drive used on the Zenith portables.
                  I think they were also used in a couple of MIDI sequencers. I forget what those were called - Quickdisk or something?

                  There were other MS-DOS formats; chiefest is that used on the NEC PC98 systems. Whether or not you want to call that "compatible" is a matter of debate, but there are versions of Windows that run on it, so it got Microsoft's blessing.
                  Yeah, that was the Japanese floppy format, right? 1.2mb on a HD 3 1/2" disk. A lot of newer PC BIOSes supported that and called it Mode 3, IIRC. I don't think I've ever actually encountered a diskette formatted like this.

                  And at least one or two systems used 720K "quad density" 5" (DSQD, 80 cylinder).
                  Tandy 2000 is the only PC clone-like-device I can think of that actually shipped with one. They were also somewhat popular as upgrades to PC/XT's, and TRS-80's since they have the same data rate as normal 360k drives, just more tracks. Other non-PC machines used them, like the AT&T 3B2. While they did sell 96tpi media specifically for these drives, you can usually use high quality 48tpi DD media. A lot of the DD 96tpi media I've seen has no hub ring, which seems odd to me.

                  It might also benefit the discussion if the term "track" was taken to mean "a annular ring of recording surface on one side of a disk" and the term "cylinder" used. The reason is that you'll sometimes see a 48 tpi 5" double-sided drive called by its manufacturer "80 track"; that is, the drive records 40 cylinders, two sides. This would also bring the terminology into agreement with hard disk terminology.
                  True, I refer to "track" as being a physical position of the head assembly. To add the number of tracks together between two sides really doesn't make any sense to me, since, in your example, the head assembly can only be in 40 different positions. But, you're right - "cylinder" is probably a better term, since there is no confusion.

                  And, believe it or not, Microsoft, for a time, also defined an 8" MS-DOS format.
                  The only machine I can think of that used this was the NEC APC. There might have been something else weird, like a Zenith Z100, which has a connector for an 8" drive and can run MSDOS, but the APC is the only machine I can think of that shipped with 8" drives and ran MSDOS.

                  I wasn't intending this to be a be-all end-all guide to floppies, although it is a lot of fun to try to think of them all. There are so many weird disk formats. Like, what about the IBM 4" DemiDiskette format? Sure, it's only a protoype, but it did physically exist. I was just trying to go for general information about common disk media used by IBM PC's and their clones, and even then, primarily trying to explain the difference between HD and DD drives and disks, and why you can't format a HD disk in a DD drive. That seems to come up an awful lot, for some reason.

                  And, if you open it up to non-PC stuff as well, there are some downright bizzare disk formats. For example, you're not going to see any hard sectored floppies in IBM PC land. At least, I don't know of any... I always like to be proven wrong. It's very hard to say a definite "this is the only thing" or "this was never done", because there were so many unusual machines. It's what makes collecting old computers so much fun.

                  -Ian

                  Comment


                    #10
                    Originally posted by RetroHacker_ View Post
                    I think they were also used in a couple of MIDI sequencers. I forget what those were called - Quickdisk or something?
                    I thought those were 3". The QuickDisk seems to be most well known as part of the Famicom Disk System...

                    Comment


                      #11
                      Originally posted by linuxlove View Post
                      I thought those were 3". The QuickDisk seems to be most well known as part of the Famicom Disk System...
                      Don't forget the 2.8" variant placed in Smith Corona Wordprocessors (really typewriters with disk storage). Same underlying disk system as the Famicom disks but placed in a different shell. In the US, the word processor form were much easier to find.

                      I am sure every disk variant ever made was adapted by someone to run on an IBM PC. Early computer users were cheap and were perfectly willing to sacrifice many hours of effort to modify discarded and discontinued storage into a usable state.

                      Comment


                        #12
                        The 3" spiral-tracked ones used on some synths were also used on some Smith Corona PWPs--in that application they're called "DataDisks" and their size is given as 2.85"

                        No, the Zenith MiniSport used 2" drives. AFAIK, it's the only one that did.

                        The 3" floppies look pretty much like 5" floppies that have been through too many warm-water wash cycles. AFAIK, they were deployed on one model of the Seequa Chameleon.

                        I've got a couple of factory packages of MS-DOS 1.25 system disks for the NEC PC8801A-16 (this must be among the rarest of the NEC PCs; apparently a PC8801 with an added 8088 CPU). The disks are 8" and 5"with exactly the same format and contents.

                        Note that some "almost PCs" such as the HP-150 and 110 as well as the DEC Rainbow have their own formats for DOS--the Rainbow in particular is a 5" QD format. And I seem to recall that the DG/One used a QD 5" format.

                        Did any MS-DOS machines use the 5" Drivetec/Kodak drives? The only systems I've seen with them have been CP/M, but I know that Kodak was flogging a 6MB model (I have one) after they picked up the ruins of Drivetec. It was definitely during the IBM PC era.
                        Reach me: vcfblackhole _at_ protonmail dot com.

                        Comment


                          #13
                          Chuck: Do you happen to know why the Kodak efforts at high capacity floppies failed? Not just the various Drivetec models but also Infoworld lists a 12 MB model manufactured by DTC (Data Technology Corporation).

                          Now, regarding the topic at hand, I did a check of Infoworld for things I vaguely remembered. July 30, 1984 issue includes a review of the Maynard Winchester System which supported the hard disk the name implies but also standard 5 1/4" floppies and what are listed as 8" drives, 1.25MB Teac drives and 2.5MB Drivetec drives. July 2, 1984 reviewed the Dynaframe which supported hard drive, 5 MB Syquest, 12 MB tape, 8inch floppies, 2.5MB floppy models from Drivetec and Amlyn plus a 6 MB Amyln 6 Discpac. Both the preceding were external chassis for the IBM PC and required a specialized card or two to interface with the PC. I never saw one so have no idea how rare these are compared to other external drive enclosures.

                          Comment


                            #14
                            Originally posted by krebizfan View Post
                            Chuck: Do you happen to know why the Kodak efforts at high capacity floppies failed? Not just the various Drivetec models but also Infoworld lists a 12 MB model manufactured by DTC (Data Technology Corporation).
                            Sure. The Drivetec/Kodak drives couldn't format their own high-density disks--you had to buy them pre-formatted and they weren't cheap. This is probably the same reason that the 3M/Imation LS-120 faded from the market. The Drivetec, like the LS-120 spun the disks at double-speed, so for people wanting fast access to conventional disks, they were attractive.

                            Note that Drivetec high-capacity disks were conventional floppies at heart, not like the LS-120 disks that used an optical overlay.

                            Drivetec used a clever two-stepper ("fine" and "coarse") positioner system and an embedded servo scheme, initially at 192 tpi and greater track densities for later drives. Very clever scheme, so the whole issue of alignment never came up.
                            Reach me: vcfblackhole _at_ protonmail dot com.

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                              #15
                              Excellent writeup!

                              I'm surprised that Chuck hasn't commented other than rolling out his usual list of obscure and stillborn formats ; I can find a few nits to pick, but all in all a pretty good writeup of most of the issues involved.

                              One thing I will comment on is "you need high density media for high density drives, and double density media for double density drives" and similar statements elsewhere. A common misconception IMHO: double density drives do indeed need double density media, but "high density" drives are usually dual (or triple) density drives and the media should really match the format being used and not the drive type, especially in 5.25" drives.

                              Right, Chuck?
                              Last edited by MikeS; June 30, 2011, 06:04 PM.

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