Change Drive Letter other Ext HD problems in Win7

[dmoyn]

New computer with Windows 7 (formerly XP). LaCie ext HD was called R drive on old computer, and ran fine. When I try to change the drive letter the usual way in Disk Mgmt, I get an error message: "The parameter is incorrect."
In the Disk Mgmt window, it says the volume (L: ) is Simple, Basic, and Healthy (Primary Partition). When I go to reassign the letter, instead of a simple L:, one window shows it as L: () (or maybe those parentheses are an odd '0'). When I mount the drive, I get a message saying there is a problem with the disk and asking if I want it repaired. And when I try to dismount it, it always says it is "in use" even when all programs are closed. I have to put computer to sleep to dismount.
Any help would be greatly appreciated

[DakshaMira]

The ‘Parameter is Incorrect’ error message mean the drive is being used. The drive letter which are assigned to system volume or boot partition cannot be changed by any mean.
Following topic can be helpful to get out from the above mentioned problem.
Windows 7: unable to change the drive letter

[dmoyn]

Thanks DM. That helped. But how do I know when/what programs are using an external HD. All programs are closed and still the safe dismount button says it is "in use". I have to shut down the computer to dismount this (and other) ext HDs. Is there a way to find the program that is using the ext HD and disengage it? (Must be something running in the background).