Inexplicably, the internal drive in my MacBook has unmounted. Using Disk Utility, I've clicked the "Mount" button, but nothing happens. I've zapped the PRAM, etc. to no avail. Disk Utility initially showed the drive as mounted when I ran the application. I don't know what's happened as I did not unmount the drive. Verify Disk and Repair Disk quit because of errors. How can I get the drive to remount? The computer is running OS X 10.4.11 with 2 mb RAM. The hardware test indicates no problems with the computer's hardware.