Display issue

[Ollie Killingback]

Up to yesterday I had a desktop (running Ubuntu 9.04) and a laptop
(running Ubuntu 9.10) in my study.  The laptop had a second monitor
attached, which I found handy.  I converted the desktop to a
file/print/web server and decided that the big screen attached would
be better used as the second monitor for my laptop, as it was no
longer needed on the server.

* With the laptop powered down I detached the second monitor and
replaced it with the larger one from the former desktop and booted.
* It refused to boot.
* I detached the 2nd monitor and it booted fine.
* I attached the 2nd monitor and opened Display and both screens went
blank.  Nothing I could do would recover.
* I detached the 2nd monitor and rebooted, and everything is normal
again, but I miss my 2nd monitor.

I could re-attach the original 2nd monitor to the laptop, but as the
other one is bigger and was working fine on the former desktop, using
it as the laptop 2nd monitor makes sense.  I see no reason for Ubuntu
to fall over because of a monitor.  It was fine with the other 2nd
monitor.

I'd be grateful for suggestions as to how to fix this.

Ollie K