Your Jaz drive is kapput, just like mine. It can read about the first 300-350 mbs and thats it. I had tried everything I could think of to get it to work to no avail. I called up the techs at Iomega and described my problem with as much detail and they basically said the drive itself has gone bad. The way to test it is to take your disk and try it in another working Jaz drive. If it reads it fine, then its definetly the drive. My problem is I dont know anyone with a Jaz drive that I could test it in.
Btw, I was also told that it was a problem with alot of Jaz drives, but not enough to cause a recall. If its still under warranty send it out to get fixed.
EDIT: I just re-read your orignal post and noticed that you had the same problem with different drives. I'm not sure what the problem can be now.