View Full Version : 3.5" disk question

October 13th, 2015, 03:29 PM
This is a dumb question, and I know I used to know the answer!

If I buy some 1.44mb or "2mb" floppys, can I format them to 800k or 400k?

Or, if I have a 400k disk, if I dd it to the 1.44mb floppy, will that be readable in older drives?

I'd be trying this with Apple II and Mac software.


October 13th, 2015, 03:42 PM
Sometimes, I've gotten away with in the past, but personally I've found most of the time DD drives will have trouble reading HD media reliably even if it's formatted DD. The media itself is physically different.

If you have a DD disk, and format it DD, in an HD drive - that should be perfectly fine.

From memory, I think you might not be able to write Apple disks with a PC? BCR vs MFM? I can't remember the details on that.

October 13th, 2015, 04:19 PM
In general, formatting a high density 3.5" disk as low density will "work", but is not perfectly reliable. But for generating a quick boot disk, or shuttling some data between systems, that will be fine.

But getting a 400K or 800K sector image on to a real disk is not a simple thing to do. Those not only use GCR encoding, but also variable bit rates. A PC floppy disk controller can NOT read or write GCR. And don't even get me started on lobotomized USB floppy drives.

A Kryoflux can read and duplicate Apple 400K/800K 3.5" disks, but currently nothing can convert sector images to stream files (at least nothing I have been able to get to work).

Now, if your Mac happens to be equipped with a 1.4MB "superdrive", those use the same MFM encoding and sector formats as PCs. (But different file systems). So you CAN write 1.44mb apple disk images with a PC floppy disk controller, or even a USB floppy drive using a tool like WinImage or Unix "dd".

October 13th, 2015, 04:50 PM
Nice. Yeah I planned on using dd in Linux. So I'm set for my Mac. I just need to sort out adtpro for the IIGS...

October 14th, 2015, 04:31 AM
Double density and high density 3.5" disks have slightly different coercivity (magnetic field strength); it's close enough that you can usually put the wrong format on the disk (i.e. 720K on an HD disk or 1.44MB on a DD disk) by either punching out or covering up the media detect hole, but it may not be reliable.

8" all formats: 300 oersteds

5.25" double density (360K): 300 oersteds
5.25" quad density (720K): 300 oersteds
5.25" high density (1.2MB): 600 oersteds

3.5" double density (720K): 665 oersteds
3.5" high density (1.44MB): 720 oersteds
3.5" extra-high density (2.88MB): 900 oersteds (unconfirmed)

p.s. I am amused that "coercivity" and "oersteds" are not in Microsoft's spelling checker dictionary.

October 14th, 2015, 07:34 AM
If you have a PC with a Deluxe Option Board, you can also read and write 400K and 800K floppies in a 3.5" PC drive.

October 14th, 2015, 11:25 AM
My experience has been that, as has been mentioned, it'll technically work and can do in a pinch for quick transfers/boot disks, but I would avoid using it for even remotely long-term storage as every disk I make this way seems to poop out after a month or two.

October 14th, 2015, 11:36 AM
I think the rule is "Your mileage may vary". I've received batches of DD 3.5" that contained some HDs written as DD from the mid-1990s. No problems--usually. But again, YMMV.

October 14th, 2015, 12:04 PM
I remember Mac 400k drives as being very finicky about the disks inserted. Not sure why since the 800k drives would cheerfully format HD disks at 800k and 400k but the 400k drive didn't like the HD disk formatted to 400k. Picking up a few DD disks seems like a cheap way to avoid frustration.