Bryan VonDeylen writes that, at the high school where he works, 20 out of the 25 internal Zip drives in their Power Mac 6500s started to fail. The drives were replaced but many of the replacements also began to fail. The main symptom was that, after inserting a cartridge, it would make a clacking sound and fail to mount. They needed to insert a paper clip in the "eject" hole just to get the cartridge out.
The solution?: Take off the two plastic bevels that cover the access to the Zip drive (not an easy task according to Bryan). This reveals the eject button (accessible on external Zip drives but covered up in the 6500). When there is no cartridge in the drive, restart the Mac and then press this button. Although I see no clear reason why this should have any effect, Bryan says that almost every drive that he has done this with now works without a problem.
Keith Arnold reports that the same cure (pressing the eject button when no cartridge is in the drive) fixed the same clacking cartridge problem he was having on an external Zip drive! This looks promising.
Live and learn. A reader writes: "This is the standard reset procedure for all Iomega Jaz and Zip drives. Iomega Tech Support passed this along about 3 months ago."
By the way, if you need help opening your 6400/6500 case, check out this web page.