A1. Run a disk repair utility on the drive.
A2. Perform permission repair on your system drive. Open the disk utility (located in /Applications/Utilities/Disk Utility.app.) Select your system drive (usually the first disk listed) and click "Repair Disk Permissions" in the "First Aid" Tab.
A3. Restart your computer. As soon as the boot up chime sounds, press and hold down the option-command-p-r key combination to reset the PRAM. Release the keys when the chime sounds a second time. Please refer to your Mac manual for more information on these procedures.