Can't access WD MYBook

Hi, I can’t access my WD MYBook Esssential 2TB thru “My Computer” on Windows Vista. The external HDD is displayed in “Device Manager” under “Disk Drives” but not shown in “Disk Management” in “Computer Management”.
I can’t access the external HDD on another dekstop running Windows 7. Please advise. Thanks.

If the hard drive is not locked and not displayed in Disk Management in different systems even though it’s displayed on the Device Manager then it would unfortunately mean the physical storage hard drive within the enclosure has failed. If so the unit should be replaced under warranty.

How to get an RMA, (Return Merchandise Authorization) or replace a defective product under warranty

What do you mean that a HDD is “locked”?
Is there something I can do to troubleshoot or maybe try to recover the files on the HDD before I submit an RMA?
I do not have a backup copy of some of the more recent files.