Backup external drive to another external drive?

WD External Drives External Drives for PC
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I backup one My Book to another using a simple batch file and the XCOPY command.  The /M option will copy only files with the archive bit set and then reset it so that it won’t get copied again next time.  I launch my backups manually by double clicking on the batch file from Windows Explorer.  I have a batch file in the top folder of each directory tree that I want to backup.

Cal