I just replaced a hard drive on a 2.4GHz MBP with a copy of Snow Leopard. It was running great for a few days but now it freezes on boot. It freezes with the Apple icon and the spinning wheel below it. It passed all the hardware tests that I ran when I booted from the Apple disc. I took out the WD Black hd and hooked it up to another Mac and ran repair permissions and I verified the disk. When I installed the hard drive back to the MBP it worked! I rebooted it once and it worked again but when I tried again it froze. What do you think is wrong? Thank you for looking.