What's in your scsi chain ? Is it properly terminated ?
Does it also happen when you connect either the cd-rom or the zip drive ?
Are you using a zip-plus drive ? When it turned out the zip plus scsi implementation was rather buggy, iomega stated that it should be the only device in the chain (check http://newmedia.com/threads/iomega/messages/435.html ).
Has it happened when you just started the sampler up ? (chips and godknowswhatother electronic components might start failing when they heat up).
scsi can sometimes be tricky... my esi32 didn't seem to have any immediate problems with my plextor 4220 in the same chain, but after a while it turned out that it would crash when saving to zip about 1 in every 5 times. When it's just my pc -> old sony cdrom -> zip -> esi it's never ever crashed.