It sounds like you have SCSI termination issues. The GVP cards give errors but still work when one end of the SCSI chain isn't properly terminated.
If the autoboot jumper is set to disabled, there is a file that you have to put in the expansion drawer to make the Amiga recognize the card. If you autoboot, the firmware on the card does that for you. The disk image with the expansion drawer file is here:
http://amiga.resource.cx/exp/gvp4008
