The 840AV was one of the first machines to have SCSI Manager 4.3 features built into the ROM.
Make sure that you do not have the SCSI Manager 4.3 extension enabled.
Launch the Control Panel for the JackHammer card and turn off the SCSI Manager 4.3 load. The JackHammer card is able to load SCSI Manager 4.3 from its ROM if the host machine lacks it.
There may be a special advanced screen you must get to to do this. It's been too long since I used the JackHammer.
I'm not certain if a SCSI Manager conflict would cause the problem you're having, but it seems like a likely place to start.
Make sure that you do not have the SCSI Manager 4.3 extension enabled.
Launch the Control Panel for the JackHammer card and turn off the SCSI Manager 4.3 load. The JackHammer card is able to load SCSI Manager 4.3 from its ROM if the host machine lacks it.
There may be a special advanced screen you must get to to do this. It's been too long since I used the JackHammer.
I'm not certain if a SCSI Manager conflict would cause the problem you're having, but it seems like a likely place to start.