The Apple support forums seem to be down for me atm so this is my next best bet! Please see attached images of my mother-in-laws 2011 21" iMac. Never had any problems with it until today. I transferred some handbraked movies from my MBP to it last night via an external USB HDD - transferred fine so I went home. She sent me this an hour ago after waking the iMac up... I wondered whether the iMac was trying to boot of the externals so she unplugged them and got the second pic instead on restart. Looks a little concerning - any ideas out there please!?