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Every time I close the lid on my macbook air, my external hard drive seems to spin up for about 5 seconds every 30 seconds, and this will go on continuously, even for several hours. The noise itself isn't too loud, so I don't really have a problem with that, but I'm worried it might damage the hard drive.

Is this normal OS X behaviour (I'm on Mountain Lion, sleep mode 3), or is it specific to my model of hard drive (Samsung 1tb USB 3)? I read that Mountain Lion will supposedly check emails and perform updates during sleep, could that be the cause?

If it doesn't harm the hard drive, I'm not too worried, but if repeated spinning up will exert extra strain, I'd like to find a way to stop this happening.

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2 Answers 2

up vote 1 down vote accepted

Unfortunately, Mac OS doesn't eject hard drives when the machine enters sleep. And since your MacBook performs some "activity" during sleep, a writing/reading process might keep the hard disks awake.

There are at least three solutions to this problem:

A) You eject the hard disks each time before you enter sleep

B) You get the Jettison App for $4.95 (http://www.stclairsoft.com/Jettison/), which automatically ejects hard disks before the MacBook enters sleep, and mounts them when the MacBook awakes. There is a 15 days free trial if you like to check if it works for you. enter image description here

C) Disable the Power Nap mode

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For anyone coming across this and trying to disable power nap mode, make sure to check both the battery and the power adapter tabs in Energy Saver, as the mode needs to be disabled in both of these sections to prevent the effect. –  nitrous Mar 30 '13 at 23:35
    
So, which solution did you end up with? –  bluewoodtree Mar 31 '13 at 0:03
    
I went for C. I didn't want to have to remount my hard drive each time I woke my laptop, and I wasn't looking to spend any money, but I might get that app at some point anyway as it sounds quite useful for when you need to unplug it during sleep. –  nitrous Mar 31 '13 at 8:05
    
I have a MacBook Mid 2010, so there is no power nap I had to bother with. But I have 3 HDs hooked up to it via USB-HUB, and ejected them separately each time when I had to take my laptop with me. But fortunately I discovered Jettison recently, which does no magic, but makes life a little bit easier. –  bluewoodtree Mar 31 '13 at 14:03
    
Does it ever fail to eject a hard drive, such as when some applications keep a lock on the hard drive, preventing the drive from being unmounted? –  nitrous Mar 31 '13 at 21:40

Try with disabled Power Nap mode!

In system preferences energy saver.

Here is my setting, but I do not have an external HD to check that.

enter image description here

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Thank you, this seems to have solved my problem :). I only accepted the other solution because it was more informative and gave an explanation of why it happens. –  nitrous Mar 30 '13 at 23:31

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