The 1st thing I would suggest is attempting to retrieve the data from the drive as a precaution.
Check out this data recovery software http://www.snapfiles...tarecovery.HTML
try Recuva 1st it has a high success rate.
Has the drive been knocked or the power turned off suddenly prior to this happening, how much
available storage space was available on the drive, any unusual noises from the drive or have you
downloaded from the net to it recently or installed new software.
Things you can try include using another USB port if you have one, disconnect and then uninstall the
drive in device manager and the software for the drive in Programs and Features (or the W7 equivalent)
and then reinstall both and finally try connecting it to a 2nd computer and see what happens.
If all else fails is the drive covered by the Hitachi 3yr warranty, it will not get your data back
but it may save you the cost of a new drive.