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How to Reset Parameter RAM (PRAM) in iMac 2015

PRAM stores certain system and device settings in a location that Mac OS X can access quickly.
Exactly which settings are stored in the computer’s PRAM varies depending on the type of
computer as well as the types of devices and drives connected. To reset PRAM:

1. Shut down the computer.

2. Locate the following keys on keyboard: Command, Option, P, and R. You will need to hold
these keys down simultaneously in Step 4.

Note: If the keyboard does not have an Option key, use the Alt key instead.
3. Press power button.

4. Immediately press and hold Command-Option-P-R keys.
Important: You must press this key combination before the gray screen appears.

5. Hold down keys until the computer restarts, and you hear the startup chime a second time.

6. Release keys.

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