My SCSI Zip 100 Model: Z100S2 Seems to have a default power-save mode (spin-down) of about 15 minutes, whether startup disk, slave or n/c at all. If you take your disconnected ZIP and pop in a disk, it should spin-down after about 15 minutes to be completely silent. If it does not, leave it in to see how long it remains spinning. Report back the results with your particular model number. As Tom Lee indicates, Zip Tools, or some other utility may well be able to alter the driver's defaults for continuous operation, as well as shortening the wait-time for power-save. The same disk on my USB drive, results in power-save within several seconds of last disk access and is absolutely silent (otherwise it does make humming and clicking noises). So I would expect the default drive behavior to assert itself when disconnected.

Page 13 of this document confirms it:

http://download.iomega.com/english/manuals/english/en181102.pdf