The Western Digital support site has an article about the MyBook Pro units and bootability with Macs; I don't have a direct link, sorry.
You can add new files to an existing backup, using "Copy Newer" or "Copy Different", but if those files are in the same location as the old with the same name, they will overwrite the old files. On top of that, the backup will no longer be bootable...
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--Dave Nanian
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