RetroHacker_
Veteran Member
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. :D 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
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
I would love to find an 8 inch disk.