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I have two external hard discs connected through firewire 800 to a Macbook Pro. Usually I only work with one, so I unmount the other, which powers down. However, after some time the unmunted hard disk powers up for a while, then powers down. What is causing this activity? Can I somehow prevent it?

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Preventing will be quite complicated, as not only a lot of system processes check for certain files on all hard drives regularly (think spotlight, time machine, etc.) and more often than not, opening a "choose file" dialog will spin up all hard drives, too, since the OS wants to leave you with a "fast" impression and thus spins up drives in advance.

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I know that happens when the drive is mounted, but in this case it is unmounted. Normal software should be unable to even know something is connected. –  Grzegorz Adam Hankiewicz Mar 3 '11 at 14:16
    
@Grzegorz I take it you are daisy chaining the drives? If so, is the one that is supposed to be unmounted and sleeping the first in line or the second? –  Asmus Mar 3 '11 at 17:48
    
The setup is macbook pro <-> firewire 800 hd (turned on) <-> firewire 400. The firewire 800 hd has a power of switch, but the 400 one doesn't, so it randomly powers up even though its not mounted or available in finder. The 800 hd is always powered on. –  Grzegorz Adam Hankiewicz Mar 3 '11 at 21:06

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