I think I have the answer, from two memebers of the List who emailed me
Since my application is for occasional backup, a performance hit is no
issue -- and my CD is only rarely used.
So, it seems I can safely install a big IDE disk in the machine -- at
worst, sharing the same device as the CD player.
Actually, I may simply transfer one of the hd's in the machine -- one
that is used very little, and is a small hd -- to another machine that's
always running, and that currently has only one (big) SCSI disk -- it's
used as the fileserver for the network.
(I wouldn't want to put the new backup disk on the fileserver -- the idea
is, to back up -- mostly -- the fileserver, in case of a catastrophic
failure of, either the whole fileserver machine, or at least the (crucial)
SCSI disk, containing most data).
Thanks much, Ms and Adam, for your timely and useful responses!