I have a feeling that I already know the answer to this question, but I'll ask anyway since a lot of people on here are much smarter than me and know things that I don't know. So, here it goes: I have a 2006 Mac Pro with a 500GB HD. I recently put in a USED WD 150GB Raptor with the intention of installing the OS and applications on that drive and using the 500GB for everything else. So I put the drive in, turned on the computer with the Mac OS Lion recovery disk USB stick plugged in, went into the disk utility and verified the drive and erased it. Then, I began to install OS X, which said it was going to take about five hours because it had to download "additional components." So I went to bed, came back in the morning and it says the installer encountered an error. I restarted the computer to try the install again, but now it won't show the 150GB in the Disk Utility. I'm assuming that the hard disk just failed during the install (my fault for buying a used disk). Could there be any other possible answer?