What to do when Mac OS X Yosemite won’t boot


Repairing a broken OSX.

There have been reports of OS X Yosemite freezing after a software update. This can happen to any version of OS X.   If your Macintosh won’t boot up firstly I’d leave it a few hours, maybe even overnight, and see if it comes good. If not, you should ring Apple as they will be able to talk you through how to fix it. Otherwise, if you are outside of your Applecare,  or want to have a go at it yourself, here’s what I’d do.

You need to boot (start up) your computer, then do a repair or reinstall of OSX. There are three possibilities that I can think of for how to start up when your computer is crashing:

1. Safe Boot (Boot just means start)

A ‘safe boot’ is for when your computer is having problems.   Safe mode runs off your normal hard disk, using your normal OS X, but it’s kind of like being under anaesthetic, only the life essential functions are running,  so you can operate on it to do some repairs. It starts the computer, but it only loads the minimum possible essential core pieces of software, then it tries to do a repair.  In safe mode lots of ’non-essential’ things don’t work. e.g. Internet. DVD player. So you only start up into safe mode, let it do what has to be done, then restart.  In
If your computer is crashing when you start.  Try to boot into safe mode. To start into safe mode you press power button, then after the chime hold down the shift key.
When you boot into safe mode it knows you are in trouble so it automatically tries to do some repairs for you. So after you have started up into safe mode, just wait a bit, then restart. That may fix your problems!

2. Recovery Partition

If your computer won’t even boot into safe mode then that’s more serious.  The old way to repair such a computer was to boot from an external hard drive, that is,  your version of OS X  is so bad that you need to give your computer a new version to start up from to get it up and going. That means plugging in a hard disk that contains a backup of your computer (or a new version of OS X)  and running from that backup while you fix the problem.
Since OS X Lion there’s a new solution to this. In OS X Lion and later OS X versions Apple have included a secret second copy of the OS X on all hard drives. This is called a ‘recovery partition’.
A recovery partition is a complete copy of the System software, that you can’t normally see or use or modify. If you boot up while you hold down Command – R your computer will ignore your normal OS X and boot into the ‘recovery partition’. From the recovery partition you can re-install the system software (while keeping all your own ‘user’ files and documents untouched). You need to be connected to the internet because when you do a re-install from the recovery partition it needs to download a lot from the internet – Gigabytes.
When you boot into recovery partition it will give you a choice of 4 options. (restore from a backup, check internet connection, reinstall OS X etc). The one you probably want is to reinstall OS X, but keeping your old files.

3. Boot off an external hard drive or thumb drive.

If you have an OS X older than Lion you won’t have a recovery partition so you will need to boot from a backup, or from a version of OS X on a thumb drive. What a great time to remind you to make sure you have your computer backed up!
Please leave a comment if you fond this article helpful or if you have any of your own advice to add.


One response to “What to do when Mac OS X Yosemite won’t boot”

  1. bks

    Thanks Wayne. A really clear and understandable post which helped me out. Lots of forums are full of comments about upgrades to Yosemite freezing half way through installing but many of them are hard to follow if you’re a bit of a computer novice. Your post was characteristically straightforward and easy to follow. Thanks again.

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