I'm fixing up a computer for my friend and wiped his hard drive clean such that I could install Yosemite on it. I formatted it, installed the hard drive via my MBP and then could easily log in and out into both my internal HDD and the external HDD with no problems. I bring the external to his MBP, same specs 13" 2010 and try to boot from the external and all I get is it hanging onthe Apple splash/boot screen. The progress bar shows anywhere from 1/3 to 1/2 loaded, but I've left it overnight once and it never moved. I've had his computer working with Yosemite to the point where I could get on the net and do normal tasks. Now it's on this screen. Don't get why, and read this was an issue in the past. I've resetted PRAM, tried to boot in safe mode, but since this HDD was freshly reformatted, don't know if that changes anything. Any tips as to how to get past this boot screen?