I ended up reformatting the SSD and trying to reinstall with the install Ubuntu 12.04 DVD. Got to where I thought the the install was just about complete (a half hour or so of downloading and installing) when I was advised:
(Not sure about the "edac" in the third line; lousy handwriting.)Installation Failed
The installer encountered an error copying files to the hard disk:
[Errno 5] input/output error: '/target/etc/skel/src/linux-3.9.8/drivers/edac'
This is often due to a faulty hard disk. It may help to check whether the hard disk is old and in need of replacement, or to move the system to a cooler invironment.
The drive is only a few months old and had been working fine until I apparently lost the "/" mountpoint. I removed and examined the outside of the drive and put it back in, re-attaching the cables with great care, and tried installing again. This time I got that message right after the "Who are you?" section before any uploading or downloading started.
Since then I have tried reformatting the target partition, but DiskUtility refuses to do it.
I wonder whether I have any options besides tossing the drive and getting a new one?
Meanwhile I had installed the Ubuntu on a partition on the drive I had previously used only for OS4, but that install turns out to be useless because I cannot set a password for it and many things I want to do require a password! (I go through the process of setting one and get told the password was set, but reboot doesn't shut down properly.)