Resetting or "Zapping" the PRAM:
PRAM (Parameter RAM) stores a number of system-wide settings that are needed when the system first loads and before it can access the hard drive. Sometimes these settings may become corrupted and the PRAM needs to be "zapped" or reset. If you are experiencing problems with your Mac retaining any of the following settings, you should reset your PRAM:
- Speaker volume
- Time zone location
- Key repeat delay and rate
- Mouse speed and double-click rate
- Monitor depth and resolution
- Default startup disk
- Startup options (verbose mode, single-user mode)
To reset the PRAM, reboot your Mac while holding the apple, option, 'p', and 'r' keys. Keep holding these keys after the first chime until the computer resets and chimes a second time. Then, you may release the keys and the computer will boot normally.
You can safely reset the PRAM as frequently as you like. Many people include this as a step when they perform routine repairs and checks such as the permissions and hard drive repairs.