From what I picked up by casual reading, there are two very similar ZIP 100 drives.

One is for parallel port and carries no jumpers on its back = bad, wont work with Macs.

The other is SCSI with jumpers between the ports = good, will work with Macs and can be used as boot volume.

If you should live in Europe, I am willing to donate a 25sub-D to 50-Centronics SCSI cable, as I own some spare ones.

If you have the money to spend, I would try to find a replacement HDD as long as they are available.

I have got a 68 pin drive in my 7100 with an 68/50pin adapter running perfectly.

Before discarding your HDD drive:

Is the 6100´s PRAM battery good?

Can you access the HDD after floppy boot?

Is your "dead" drive still seen on the bus?

Did you try to low-level format it?