When I run disk utility and repair permissions with the iMac HD everything verifies and repairs fine, when I try to repair my external 250 Lacie HD this message appears:

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Verifying volume “Luton 250GB”
1 volume checked
0 HFS volumes verified
1 volume failed verification

Verify volume failed with error Could not unmount disk (-10000)
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and a pop up box says "this disk could not be unmounted"..

whats the problem here? how can i fix this?