[Matts]

A quick update:

The repair shop called, we're getting our iMac back later today.

They said that there were some bad log files on the internal (boot) drive that caused the malfunction of the computer.
Nothing wrong with the motherboard, the memory or anything else.

They have now wiped the drive (I have a clone from when it initially worked beautifully; right after I'd installed and configured it) and re-installed the OS.

Sounds (more than) strange to me that bad log files on the internal drive could prevent the iMac from booting from an external drive, and while attempting to do so, even cause a kernel panic, don't you think?

I try to find out more about the log files, and we'll see how the iMac behaves on return - and after I have restored from my clone (which I sincerely hope doesn't include any bad log files!).

-matts