"My Dad just got a new iMac.
It seems that Lion will not save files to a USB Zip drive? I know Zip discs are ancient technology, but he likes using his Zip discs."
Try these steps, in the EXACT order presented.
1. Eject a ZIP disk if one is in the drive
2. Power down the computer (all the way down to "off")
3. Have a ZIP disk "in hand" and ready to put into the drive
4. Press the power-on button on the iMac, and immediately afterwards, push the ZIP disk into the drive.
5. Let the Mac boot.
6. When you get to the finder, does the ZIP disk "show up" on the desktop? It may, or it may not.
Notes for the above:
All ZIP disks have an "on-disk" driver that will load at boot time (presuming the OS doesn't already have drivers for the ZIP). I'm going to _guess_ that a new iMac has Lion pre-installed, and that Lion probably doesn't have an included driver for the Iomega ZIP drive. In that case, at boot time, the OS _may_ recognize that there is a ZIP drive physically connected, and it _may_ be able to locate and load the Iomega "on-disk driver".
If that happens, the ZIP disk may mount on the desktop. Once the drive is loaded, you can eject the disk (just drag its icon to the trash to dismount it), and the on-disk driver stays loaded until the next reboot. You _should_ be able to put another ZIP disk into the drive, and the iMac should find it and mount it.
No promises on the above, because the ZIP is an older technology.
Another thing to try (again, do it in the order presented):
1. With the ZIP drive connected, open "Disk Utility" (it's in the "utilities" folder).
2. In DU's window, you should see all connected devices
3. Do you see the presence of the ZIP drive? (it may or may not show a disk present)?
4. If you see the presence of the ZIP, try clicking on it and re-initializing it for "HFS+, with journaling ON"? Any help?
Questions:
Was the ZIP drive in question previously used on a PC?
Could the ZIP disks be in a non-Mac format?