Found this thread which might be the cause of failure to unmount at
least - http://ubuntuforums.org/showthread.php?t=393002 - apparently a
hidden trash file is the culprit.

As for the Adata drive that won't mount - does it mount on other
systems and also does it have an external power source ? I not, i.e.
it's a 2.5" laptop drive, then sometimes the USB port cannot supply
enough power to spin the drive and there is usually a second USB plug
that needs to be plugged into another USB port to provide the extra
power.