Looks like my 1TB hard drive is toast in my 2 year old iMac. I am running a small 250GB external hard drive right now to boot it. My question is, can I just buy a good external hard drive and run Mac OS from that instead of taking the iMac apart and replacing the internal drive? If so, what is a good, reliable drive to run for this? I already have a 2TB WD external HD for my backup. I don't mind having another drive on my desk.