External hard drives UNMOUNT when verifying with disk utility

I have two external Lacie hard drives attached to my iMac. One is just for general backup purposes. The other is for Time Machine.
Today when I got home, for some reason the Time Machine drive was not appearing on my desktop. I rebooted the computer, the Time Machine drive reappeared, I ran a back up, everything seemed to be working fine.
To make sure the disk was ok - I ran disk utility. When I selected "verify drive" from disk utility - the disk suddenly UNMOUNTED itself, at the point of cataloging the hard drive, and the verification process stalled. What was originally a 1 minute verification suddenly changed to a 17 minute, then a 33 minute, etc.
I tried booting up in recovery mode (Command -R at startup) and the same thing happened. Are the drives going bad?

Actually - I think when disk utility verifies the disk - disk utility itself actually "unmounts" the disk during the verification process. Probably so the disk cannot be accessed by anything else but disk utility.
I feel sort of silly, because what I thought was "stalling" was really just the disk utility going through its proper verification of the catalog file, multi-linked file, directories, etc.

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Hello,
I have just experienced the same thing!
Although my external drive shows in Disk Utility, it does not show in Finder.
And the Verify/Repair buttons are also greyed out.
Also, although the drive is 2TB. DU shows it as having Total Capacity of only 4.14GB! (Which should be the Free Space).
I was wondering if you solved your problem, & if so, would you share your solution.
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matthewfromlincoln wrote:
I've been using a western digital my book (500gb) external hard drive connected to my Airport Extreme for TIme Machine Backups.
I hate to have to tell you this, but backing-up that way is unreliable, and not supported by Apple. See Using Time Machine with an Airport Extreme Air Disk.
But, as ds store says, the disk is likely dead.

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Maxtor Support..
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..Step by Step to fix your Mac
https://discussions.apple.com/community/notebooks/macbook_pro?view=documents#/

I get the following error when formatting a external hard drive. Partition failed with the error: POSIX reports: The operation couldn't be completed. Cannot allocate memory

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Partition failed with the error:
POSIX reports: The operation couldn’t be completed. Cannot allocate memory
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First, try a system reset. It cures many ills and it's quick, easy and harmless...
Hold down the on/off switch and the Home button simultaneously until the screen blacks out or you see the Apple logo. Ignore the "Slide to power off" text if it appears. You will not lose any apps, data, music, movies, settings, etc.
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Do not worry. It is probably a permission you should ignore, but you should check that the permission appears in this site > http://support.apple.com/kb/TS1448?viewlocale=en_US&locale=en_US

External hard drives UNMOUNT when verifying with disk utility

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