Updating to 1.20 won't solve your problem - that OS is for the MCD (card reader) version, so if you have a floppy installed DO NOT UPGRADE! All OS's should work fine, but for the standard 2xl
1.14 is the last OS release prior to MCD
http://www.akaipro.com/supportos.php#MPC2000XL
Is the drive sitting in between the MPC and the computer? If so, you'll have to disconnect from the 'puter to avoid a sharing violation.
Do you have other scsi devices in the chain or just a zip on its own? With scsi, the last unit in the chain MUST be terminated. Zip drives tend to have a termination switch built in, whereas hard drives and CDs often don't.
Check with another disk in case the one you're using is corrupted. If the disk content is backed-up elsewhere, try using the MPC to format the disk. What's the zip currently formatted to? Not sure if Mac format is accepted or not as I don't have a Mac. Should be in the manual?