i copied the whole hard drive of an old imac of mine from 2006 (10.5.8) to a seagate usb hard drive so that i could access the files and even boot it up if i wanted to. i have another imac from 2006 as well as macbook from 2007 (both running 10.6.8) and i get the choose which disk to boot up option as always. i select the external drive and after a few second it turns off and then the computer as well. anyone know what could be the reason for this? it worked fine a few months ago.