Hello all, last night I installed a new 500Gb 7200 RPM Seagate Hard drive in to my friends aluminum macbook. I don't know if it is the 2008 or 2009 model. I did everything correctly and when it came time to turn his MB back on and hold down option nothing came up except the OSX install disc. I then tried to click the disc to allow me to get in to disc utilities to format the HD. But all of a sudden his macbook restarts; then I repeat the process and the MB restarts again and again.... Finally I took the hard drive out of his MB and put it into my 2006 MB thinking it was faulty. It wasn't and I was able to format the HD and install the OS on my MB. I put his old HD back into his MB and it booted right up so I knew a pin hadn't been bent. We set up everything that needed to be set up using my MB until the whole process had been completed. Finally we put the new HD back into his MB and booted it up. Waited about 20 seconds.... Nothing! We were shocked and confused. So we took out that HD and put his old one back in and booted it up and it boot right up. What could be wrong? Are there hardware/firmware restrictions that aren't allowing the hard drive to work? Why would the hard drive work in my MB but not his? I am just confused because I've never run into this problem before. Any help would be great. Thanks!!