I woke my Mac Pro up from sleep mode and I got an error message saying something along the lines of a disk not being ejected properly. I dismissed the message at the time but soon realized that my fourth hard drive in the computer has disappeared! I used this hard drive as my Time Machine backup, and also used it for BootCamp with Windows Vista. Now the partition is gone too. Does anyone know what might be the problem? I checked Disk Utility and it's not there. I also pulled the hard drive out and put it back in - nothing has worked.