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There is an option "Put the hard disk(s) to sleep when possible" in Mac OS X's preferences:

Is it possible to configure which disks Mac OS X can put into a sleepy state? For example, I want to leave only the drives which are installed in my laptop, but not the drives that are connected via USB. How can I reach this?

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Why not the USB drives? – duci9y May 4 '13 at 9:23
@duci9y my external 2.5" USB hard-drive cannot wake up (doesn't start spinning) after Mac OS X puts it into sleep. – Sunny Reborn Pony May 4 '13 at 11:27
That's probably an isolated issue, as my external drives wake up, AFAIK. Hope someone is able to help you. – duci9y May 4 '13 at 12:56
System starts working veeery slow, Finder doesn't open at all after Mac OS sent the external harddrive to sleep. The only solution is to unsafely eject a USB-cable from MacBook. – Sunny Reborn Pony May 6 '13 at 16:05
It is too hard for me to find the reason of this issue.So it would be much easier to configure which disks the system can put into sleep. – Sunny Reborn Pony May 9 '13 at 19:40

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