Epson drives (Was: C64-C128 CP-M Cartridge Interest

Chuck Guzis cclist at
Mon Dec 27 13:42:26 CST 2010

On 27 Dec 2010 at 19:19, Tony Duell wrote:

> the original HP 3.5" drive unit -- the 9121 was designed ot be exactly
> compatible with the 82901 5.25" drive unit. The latter was a 35
> cylinder 2 head deevce, so 70 physcial tracks. The 3.5" drive
> therefore only used 70 physicla tracks too. But HP also had this idea
> f having 'spare' tracks so that slightly defective disks would appear
> perfect to the user. And this was handled by the drive unit. There's
> soem kind of infromation stored o nthe inntermost track whih gives the
> track replacemtn indromation, disk usage count, etc. No hP maual that
> I've seen has any real details of this.

I wonder if it was universally implemented on HP gear.  I've seen HP 
150 Series I floppies with clusters marked bad the usual MS-DOS way.  
You'd think that such a thing would be impossible with an active 
"alternate track" scheme.


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