Safe Sleep vs. MacBook HDD

The is a little known feature on MacBooks that is quite handy, but if you don’t play by the rules it can get you in trouble. I learned this the hard way so I thought I’d write about it so that others might not have to.

Safe Sleep is a cool little feature that is meant to make your life easier by saving the current state of your MacBook – be it standard, Pro, or Air – before it loses power. Safe Sleep is engaged whenever you close the lid to your MacBook or when it automatically puts itself to sleep because it’s battery is getting low. This is a great thing because you might run out of battery while in the middle of working on something important, or need to swap out your battery in the middle of a long flight when it dies in the middle of Return of The King.

The way that Safe Sleep saves your current state is by writing the current contents of the RAM onto a temporary cache on your hard drive. OK? Bear with me.

There is another great feature standard to MacBooks called SMS (Sudden Motion Sensor). This uses accelerometers to detect sudden movements, like your MacBook falling off the top of your car as you begin your drive home from work, and to lock the hard drive arm so it doesn’t damage the hard drive if it is writing to disk at the time. The accelerometers also have other uses like MacSaber, but that’s not really within the topic of this post.

Sounds great, right? MacBooks are chock full of great features! So what’s the problem? Here’s where it gets . . . dangerous.

While your MacBook is in the process of going to sleep and Safe Sleep is writing its temporary cache SMS is not, I repeat NOT functional. This means that if you move your MacBook around before it is fully asleep you can cause minor corruption on your hard drive. If you do this once or twice it’s not going to be a big deal, but if you do it say an average of 4 times a day for several months, as I did, you’re going to eventually kill your hard drive. 

I know, I did this twice before I learned about this and once more after; it was too late. Since I got my fourth hard drive I haven’t had any trouble. So what’s the secret?

You have to wait until you see the sleep light “snoring” before you move your MacBook. That’s it. Why doesn’t Apple tell us about this? It’s kind of a big deal, right? Well, check your User Guide (PDF) – it’s all right there on page 15.

I know this was a major facepalm for me – hopefully I’ve saved you living through the same trouble.

And yes, you can deactivate Safe Sleep in Terminal, but I’m not going to bother including the code as I don’t advocate turning it off. It IS cool – you just have to play by the rules.