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After turning time machine on and then off again for an external volume, the disk will spontaneously unmount and takes around 30 seconds to list directories. Disk utility can't find anything wrong with it. Does anyone have any idea what could be the issue?

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External drive enclosures often don't support providing SMART information about the drive. Can you take the drive and plug it into a desktop machine to check its smart settings? – Daniel Beck Jan 3 '11 at 15:24
What external drive is it? Western Digital green drives (for example) turn off after a period of time to "save energy" and there's no way around it. – user3463 Jan 3 '11 at 16:03
It's a samsung spin point. SMART informations shows nothing unusual. I think it maybe a permissions issue. It works ok playing music tracks from it. But for example if I try and empty the trash folder it hangs then unmounts. – Leda Jan 3 '11 at 16:28
up vote 0 down vote accepted

how old is the drive?

Also try smartmontools

sounds to me like a drive soon to go bad. =/

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Disk Utility does SMART checking and says the disk is fine. – Daniel Beck Jan 3 '11 at 15:23
SMART saying the disk is fine doesn't mean much -- I once ran into a disk that sounded like a coffee grinder, but wasn't reporting any SMART errors. – Gordon Davisson Jan 3 '11 at 19:44

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