Charlieman's suggestion will allow you to mount a zip250 if you first boot from another disk. However, if you want to use a zip drive in lieu of a hard drive altogether, you will still need to solve the driver problem in a way that permits booting from that zip. I do not know how to make that work in system 6 with a zip250.

Btw, a zip250 is backwards compatible with a zip100, but writing a 100MB disk in a 250MB zip drive is *very* slow (about 3x-4x) because the narrower 250MB head has to make 3 passes (!) to erase and write 100MB tracks.

Since I have a pile of 100MB zip drives and disks, that's pretty much all I use. I do have a couple 250MB drives, but they get very little use because of that write speed issue with my large-ish collection of 100MB disks.