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I have updated my MacBook pro Yosemite to El Captain. During the update my macbook put an error message saying it failed to load the update. So I rebooted the MacBook now he puts me this error message (see screenshot) and restarts repeatedly.

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Do you have any ideas?

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  1. First get any files you need backed up. Check your backup drive if you're not sure you have one. The Mac can be connected to another in target disk mode or you could boot to recovery and reinstall an OS onto a USB drive (even thumb drive if it's large enough) to get backup completed.
  2. Power off and start in safe mode - hold shift when starting. If you get there, you can try the patch again, try a normal boot again and/or back up files and then try reinstalling the OS.
  3. Boot to recovery and reinstall the OS

These are in increasing levels of time, but none of the two options presented are designed to delete user files or apps, so you would only lose files if the cause of the initial difficulties were filesystem corruption or hardware errors.

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Just take any working bootable external Mac disk (Firewire is my favorite, flash drive is fine, USB works too in some cases - not to forget Thunderbolt) and reboot your MacBook Pro from the external disk. To boot from an external disk you'll need to hold the option key down while starting up.

Now that you're up and running again:

  1. copy any files off the boot drive which you don't have backed up.
  2. do a clean install of the OS
  3. test that you can boot now.
  4. copy back any missing data files
  5. reinstall any applications which aren't working or have gone missing.

Enjoy troublefree computing again. Be happy that Apple still allows booting from external drives (and usually from other computers) unlike those poor souls over in the Windows world who can only boot from an install disk.

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