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

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. The single-0sided drives in the earlier 9133 uniuts are the same as te 9121 (to the exptent that the firmware EPROM on the controler board is the same)..The double-sided ones are different, I think you get something like 77 user cylinders on thsoe (and still some bad track informaiton on cylidner 79.). -tony