It does seem to work rather better with an external PSU connected to the scsi2sd and a different SCSI cable. I don't seem to be able to get it to respond to anything other than SCSI ID 0, so I wasn't immediately able to get it running at the same time as my old hard disk, but I guess I can swap the hard drive to ID 1 (assuming I can find some 2mm jumpers somewhere) and then it won't matter if the scsi2sd is stuck at zero.
I think I need to spend some time now fixing up my A3000 so I have a reliable terminal to connect to the fileserver. It's a bit tedious having half the keyboard not working properly.
for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)