The Macintosh Portable can access and use any external SCSI device that a
Macintosh Plus can access. There may be an issue with the SCSI driver on
the hard drive and compatibility with the Portable boot ROMs, but we do
not have any information on this problem.
The error message you gave above is roughly equivalent to an "ID=02" --
meaning that you can't really use it seriously to diagnose the problem. It
does tell you that some piece of recently-loaded software got clobbered,
but it's a guess to say whether it was due to software corruption, SCSI
signal problems, incompatible drivers, or all of the above.
The best thing to do when getting this kind of error message is contact
the third-party's service organization for any possible information on
driver compatibility.