macbook pro no reconoce pantalla externa

1. repair permissions en disk utility

2.OS X Yosemite: Reset your computer’s PRAM

If your Mac doesn’t seem to be remembering your speaker volume, display resolution, or the disk it should be using during startup, you may need to reset your computer’s PRAM.

Resetting PRAM may change some system settings and preferences. Use System Preferences to restore your settings.

Important:   To print these instructions, click the Share button in the Help window, then choose Print.

1. Choose Apple menu > Shut Down.
2. Locate the following keys on the keyboard: Option, Command (⌘), P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on your Mac.
4. Immediately press and hold the Option-Command-P-R keys. You must press this key combination before the gray screen appears.
   Continue holding the keys down until your Mac restarts, and you hear the startup sound for the second time.
5. Release the keys.

After resetting your computer’s PRAM, you may need to update your speaker volume, display resolution, and startup disk preferences. For more information about PRAM, see the Apple Support article About NVRAM and PRAM.