Ah, so I guess it might be my SCSI controller, then.
I managed to put a few SCSI tools on my 6.0.8 start floppy, and the IDs of my external devices show up in a very early version (1987) of SCSI Probe, but I'm not able to mount the external hard disk. With Mount Everything (a misnomer, if ever I heard one) the IDs are not explicitly detected, but the scanning process pauses for a few seconds at the IDs on which I have the devices. So, there is some kind of data transfer through the SCSI bus. Unfortunately I'm stuck with very outdated utilities since the computer refuses to boot System 7 from a floppy.
I managed to put a few SCSI tools on my 6.0.8 start floppy, and the IDs of my external devices show up in a very early version (1987) of SCSI Probe, but I'm not able to mount the external hard disk. With Mount Everything (a misnomer, if ever I heard one) the IDs are not explicitly detected, but the scanning process pauses for a few seconds at the IDs on which I have the devices. So, there is some kind of data transfer through the SCSI bus. Unfortunately I'm stuck with very outdated utilities since the computer refuses to boot System 7 from a floppy.