According to the official documentation, failure to use the IIfx terminator
can not only affect SCSI bus performance but can also damage the bus.
Whether this is your problem or not I don't know. I have had sporadic SCSI
problems with my IIfx since I bought it. (I cannot connect more than three
devices, fourth one causes major problems).

First thing to do is to try to reformat your drive on someone elses system.
If you continue to get errors it is probably the drive. If it formats fine
then I would try to format it on your system with no externals. If this 
fails then the SCSI controller on your IIfx needs repair/replacement.

Hope this helps.
 