I checked all the cables and removed the external devices. I put a
terminator on the back of the machine. The output of "probe-scsi-all"
is still erratic. Sometimes it shows nothing, and other times it shows
the devices, but just as "Target 0", "Target 1" etc., then hangs. It
seems to do these alternately. Removing the CD-Rom just removes it from
the list of devices found. The behaviour is a bit like what happened
when I put a differential SCSI terminator on the back once and it hung
the SCSI bus. But I've checked the cables and it's been working fine
with these devices for over a year.

"test-all" gives the following message for the SCSI controller. It
isn't explicitly a failure message. I'm not sure what it means:

/sbus@1f,0/SUNW,fas@e,8800000
CE DMA fill from address ffeea000 for 80 bytes

Thanks for the help,

Chris