switch users --> black screen, no response [in 12.04]

[rikona]

This has happened a few times: switching users gives me a black
screen, no response to anything, keys or mouse. alc-ctl f(whatever)
does nothing. Assuming that it actually is switching, entering the
password also gives nothing. So far, in past occurrences, I have not
found a way to recover except pull the plug.

But - this time I have a couple of USB drives attached and have been
copying files to/from these disks, so a final sync may not have been
done. A power-off shutdown might damage the backup disks, which would
be a serious problem.

My primary concern now is saving the USB disks. I can confirm that the
box is still running - a Win2k box on the net can see and access
files, so it's not a complete crash.

What can I do to preserve the USB's and/or restore the box to
operational status?

If there is no way, is there a way to repair the damaged USB disks,
short of another full recovery process?

-- 
Best regards,
 rikona