On my new MBP (2011 2ghz 15") I'm unable to boot from external bootable clones. I have a FireWire drive and a USB. Both bootable clones I made from 2 other macs recently (2010 iMac and 2010 MBP). I turn on my MBP, hold down the alt key, I get the drive selection, select it, then boot from it, but maybe 20 seconds later the ext drive stops spinning and the MBP just stays on the white screen with the Apple logo. Same exact thing with both external drives. Ideas?