Few drives >1GB will boot the Plus, in my experience, thanks to the not-quite-compliant SCSI implementation of the Plus (it was the first, after all).
As for the Zip problem, though, you should be able to get that to work with some more fiddling. Here are some things to try:
1) Experiment with different power-on sequences. Sometimes it matters whether the boot volume is powered up before or after the computer. Also experiment with "disk inserted prior to power-up" and "disk inserted after power-up" (just in case that makes a difference with your setup).
2) A terminator is necessary, but the Plus does not supply terminator power. A device external to the Plus has to supply that power. Depending again on power sequencing, it could be that the bus is not properly terminated during the time the Plus queries the bus.
3) Try installing a fresh v4.2 driver. If that disk has been used in a system with a more recent version, the driver might have been modified without your noticing.