"intelligent" disk drives

On 18 Nov 2007 at 16:41, Roy J. Tellason wrote: Maybe you mean perpendicular recording (PMR) that was used in the 2.88MB 3.5" floppies. Very unreliable. DSHD 3.5" diskettes are rated somewhere around 17-18 kbpi density; an LS-120 exhibits about 45 kbpi--the real capacity gain is achieved by packing more tracks on the disk. The LS-120 does this with an overlay that guides as IR laser servo setup to get 1376 tracks per side. The disk is spun faster to improve S/N. The Zip drives use an "embedded" servo for positioning; that is, the servo information is part of the normal track. In addition, Zip disks have extra tracks called "Z tracks" which, among other things, track and hide bad sectors so that every disk looks more-or-less 100% "good". You cannot low-level format a Zip disk--if you degauss one, it's trash, since the servo information is written at the factory and nowhere else. The Iomega drives were not the first floppies to use embedded servo-- Drivetek was--and obtained about 2.8MB and later 6.4MB on a 5.25" diskette. If you happen to run across one of the 2.88MB ones, try reading it in a 1.2MB drive--you'll find that you can see every other track; thus, the Drivetek recorded 160 tracks per side. It used a dual-positioner technology; a "coarse" positioner and a "fine" positioner. But like all embedded-servo drives, the media was factory-formatted. (BTW, Wikipedia is utterly silent on Drivetek). Hope this helps. Cheers, Chuck