Computer was humming away happily. It had downloaded a kernel update eariler in the week and been asking for a reboot since. This morning I relented.
When the computer powered back up it made a loud repeated clicking noise which I tracked down to my hard drive by opening the cover. Since then I can't boot the computer - it keeps failing out to netboot (which goes nowhere).
Based on a quick google I think the disk is history.
Does anyone know of a way to recover the disk? I realise I'm probably SOL but it's worth asking.
Thanks for the answers. All very interesting and informative.
Nothing on the drive is worth the cost of a professional recovery service and my backups cover the important bits anyway. I was just hopeful that I might be able to avoid the nuissance of a rebuild on a new disk.