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I was wondering if Mac OS X tells my USB hard drive to park its reading head or to spind down or something similar when I unmount it (i.e. press eject in Finder). Any ideas?

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It's not related to operating system at all. When modern hard disk drive loses power, it will park its head automatically while the disk is spinning down. For the system to work, no power is required. In the end, your HDD's head will be parked unless it somehow jams in non-parked position.

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I think he was asking about long periods of inactivity, therefore the example of when the disk is unmounted -- it's not losing power in that case either. –  Daniel Beck Mar 13 '11 at 22:45

That depends on the controller of your USB disk. Basically, OS X sends a signal to the device telling it to suspend. What the disk controller does is up to its manufacturer.

I have a Maxtor drive that does not shut down or park its head. Then again, I also own a LaCie drive that goes into standby. It really depends.

Edit: I know that head parking is not the same as spinning down, but I wouldn't say that OS X tells the drive when to park.

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I thought that head parking was automatic on drives since about 1980? –  Majenko Mar 13 '11 at 20:25
    
The question was whether OS X tells the drive to do that, and it doesn't. Maybe it does on the internal drive (you know, shock resistance). –  slhck Mar 13 '11 at 20:30
    
I edited the question to reflect that this is about shutting down the drive safely. –  lhahne Mar 13 '11 at 20:47
    
As I understood it it's been something the drive has done by iself since some considerable time back and the OS no longer has to 'park' the heads (I remember a 'park' command in DOS 3.something) –  Majenko Mar 13 '11 at 21:03
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Yeah, that's true, you could do it manually a long time ago. @lhahne, if your concern is that the drive is safe after ejecting, I think you're only able to tell from the fact that it's not spinning anymore. –  slhck Mar 13 '11 at 21:07

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