[rescue] 3.5" floppy drive question(s)

James Birdsall jwb-tech at picarefy.com
Sat Oct 11 12:15:25 CDT 2008

Curious George wrote:
> --- On Sat, 10/11/08, Curtis H. Wilbar Jr. <rescue at hawkmountain.net>
>> Was wondering if 3.5" HD floppy drives can be jumpered/used in
>> a system that only supports 720K, using them as 720K only
>> drives ?
> AFAIK, if you cover up the hole in the medium that indicates
> the medium to be "HD", the drive will sense it as a 720K
> floppy and act accordingly.
> *However*, you will probably get less than ideal results this way.
> IIRC, the data track on a 720K drive is wider than the *head* on
> a HD drive.
That was true for 5.25" drives, where they added more tracks (from 40 to 
80) as well as more sectors (9 to 15). For 3.5" drives, the number of 
tracks is constant (80) but they doubled the number of sectors instead 
(9 to 18). The width of the head could be different too, but there's no 
obvious reason why it should be. Everybody knew about the problems using 
a 1.2M drive to make 360K disks, but I don't recall hearing anything 
about similar problems with 1.44 and 720.

(Speaking of 3B1's, they had their own special wrinkle on this. They 
could format 10 sectors per track instead of 9, boosting the capacity of 
a 5.25" DD disk to 400K.)


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