Conclusion: SCSI on the Powerbook 1400 is dead. Since this PB probably was NEVER connected to a SCSI device in its entire life, the problem has been there for 10 years.
I wonder if opening up the case and looking for a loose or broken connection between the SCSI port and the main board will solve the problem. I wonder what kind of voltages, on what pins, I should look for to fix this.
Or … I could live without SCSI and do all my transfers and backups via Ethernet, which DOES work.
Still, I’d love to fix the SCSI. I’ve got four SCSI drives (all free) — two ZIPs, one hard disk, one Jaz — and nothing to plug them into. On the other hand, if you’ve ever seen the cables that plug into these drives, especially the ones that mate with the Powerbook’s weird-looking square plug, you should immediately give thanks for the thin, easily plugged-in cables for Ethernet and, of course, USB.