[X4U] OT: accessing old floppy disks

Michael Winter winter at mac.com
Fri Mar 25 07:02:41 PST 2005


On Mar 25, 2005, at 8:35 AM, Jim Colgate wrote:

> Just a post-script to this tale. I had a box of heterogeneous floppies 
> and
> went through the same trial and error. Interestingly, The USB floppy 
> drives
> I tried would read an IBM PC formatted 720K floppy but would not 
> recognize
> the Macintosh 800K ones. Go figure!

That actually makes sense. The way Apple was able to get more data on 
the same disk, was that they used a drive that would slow down the 
rotational speed when reading/writing to the outer "rings" of the disk. 
That means they could pack more information there. The IBM PC formatted 
disks don't rely on this trick, thus the lower capacity. All the rings, 
inner to outer, have same amount of information.

So the bottom line is, in order to read the 800k Mac disks, you need a 
drive/controller combination that can handle changing the rotational 
speed of the disk. Since it costs more to do this, manufacturers left 
that feature off the USB drives since the HD disks don't rely on this 
trick and most people had switched from the DD disks.

-Mike



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