Recent models of the Jaz drive (with firmware version H.71 and later) are shipped in a so-called "write-verify" mode. This mode causes problems for operating systems (like Linux) that can't run Iomega's proprietary drivers.
I (Chris) have been in touch with Iomega tech support, and they provided me with the low-level commands that allegedly get the Jaz drive into the correct mode. However, the commands they provided don't seem to work (probably (ab)user error, knowing my level of SCSI programming knowledge).
In the meantime, the only thing to do is find a Mac or PC with a SCSI interface and use Iomega's Jaz Tools software to disable the write verify mode. This can also be done with a registered copy of Shapeshifter on the Amiga. On a PC, you will need to use the Windows version of Jaz Tools to disable write-verify (the DOS version of the tools won't do it).
More information about the Jaz drive and Linux (including the Jaz mini-HOWTO) can be found at Bob Willmot's Jaz-Linux information page.