[OT] Help with ancient Mac

[mark at buttery.org (Mark J. Dulcey)]

dsr at tao.merseine.nu wrote:
> 
> The floppies aren't the problem; it's finding a working drive to
> format and write them. Macs used 3.5" floppies formatted to
> 800KB, rather than 720KB: the tracks were written by a narrower
> head. IIRC, Amigas oould deal with the same format.

The 800K floppies were written with the same head design as 720K 
floppies; the reason you got 800K instead of 400K is that the disks were 
double-sided. It was the data recording technique that made Mac floppies 
different from everyone else's. They used both a different bit encoding 
(GCR rather than MFM, if you must know), and variable data rate 
recording (the larger outer tracks held more data than the small inner 
ones). (By the way, CDs, DVDs, and all modern hard disks also use that 
trick to squeeze in more data.) On the original 400K drives, the 
variable data rate thing was done by actually changing the rotational 
speed of the disk drive; that's why the sound of the drive changed as it 
read different parts of the disk. On the 800K version, it was done by 
changing the data rate clock of the disk controller, eliminating the 
need for a disk drive that was mechanically different from everybody 
else's disk drives.

At one time, Central Point Software (later bought out by 
Norton/Symantec) sold the Copy II PC Option Board. The primary purpose 
of the thing was duplicating copy-protected floppies. But it also came 
with software that could read and write Macintosh floppies, which was 
actually the main reason I bought it; I was doing the BCS Atari 
newsletter at the time, and I wanted to be able to read the Mac disks 
that I had put some of the newsletters on. I had that board at one time, 
but I don't think I still do; in any case, I no longer have any 
computers with ISA slots.

The Amiga used yet another native format that squeezed 880K onto the 
same disk. But the machine could also read the 720K PC format, and the 
400K and 800K Macintosh formats.

When the 1.44MB floppies came along, everybody standardized on the same 
sector size and recording format. It's done by squeezing more data onto 
each track, rather than using narrower heads; the new disks have a 
different magnetic coating. The Mac continued to use its own file 
system, so you still can't read those floppies on a Windows box unless 
you install a special driver. That's also true for Linux, but the 
software is free.

It would be tough to find any new 720K/800K floppy disks now; all the 
floppies I've seen in stores any time recently are 1.44MB disks. Using 
the 1.44MB disks and formatting them in 800K format would work to a 
limited extent. (You'll get the best results by bulk-erasing the 1.44MB 
disk before doing the 800K format, unless you can find the rare floppy 
that's not pre-formatted at the factory.) The old machine with the 800K 
drive probably won't be able to write to the disk (the new magnetic 
coating requires a stronger magnetic field to write, and the old drive 
probably won't be up to the job), but should be able to read data 
written by the new drive.

Finally, you'll need a Mac old enough to have a built-in floppy drive to 
do any of this. I don't think the USB drives that people use with new 
Macs bother with any backward compatibility for the older floppy formats.