Updated Hard Disk. Mac Mini. Unmounted,

Hi there.

I use a mac mini as a media hub for my apple tv's.
I recently upgraded the disk to 1TB. Everything went well. I installed Lion and away we went.

I use the Mac Mini via screen sharing on my Macbook pro. Sometimes when I go to log in the mac mini isn't shown. It seems to have locked up.

So I reboot it. However twice when this has happened. OSX won't boot up. In disk util (booted from the osx disk) it shows the 1TB drive unmounted. At this point I have to reformat and start again.

Is there something I can do apart from hold the power switch in and reboot? Should that cause OSX to fail?

Any ideas would be most welcomed.

Regards,
Simon