How to stop explorer.exe from keeping files open

I have a Windows 2003 server machine, this machine is backed up by a nightly scheduled task that copies files and folders to a portable USB hard drive (encrypted using TrueCrypt)

Every week the portable drive is takes off-site and another drive is connected (we currently rotate between 2 portable drives).

The problem is that every week when we rotate the drives we can't dismount the connected drive because of some random file or folder that is open, the process holding the file open is always explorer.exe.

If I close the handle by using ProcessExplorer everything works – the volume is dismounted cleanly and explorer continues to work normally.

I want to know how to stop explorer from keeping the file open in the first place, I suspect some 3rd party code that is loaded into explorer is the problem but I don't know how to track it down.

This is a production server, so installing debugging tools or doing anything that will take the server off-line for more than a few minutes is not acceptable.

Thanks

EDIT: Thank you for suggesting unlocker but I want to prevent getting into the locked file situation in the first place, this is a production server and I don't want mess up the explorer process.

Another Edit Is you want to know why I don't want unloacking tools just read this http://technet.microsoft.com/en-us/magazine/2009.04.windowsconfidential.aspx