You can close this one - looks like hardware.

When booting with the covers off, the diagnostic LED on the controller
showed that the bus was hung while trying to talk to sd3.  There are
several causes listed in the docs, the only  one consistent with the
problem on the 1542B and working ok on the 1542CF was faulty
termination - probably the terminators on the 1542B itself.

I've now swapped the controllers and the i486 is now running off the
1542B (with an external terminator) and the P166 is up and running
with sd3 hanging off the 1542CF.

--sjg