It's probably something to do with special permissions that Apple puts on its Time Machine backups.backupdb. I tried copying one of the backup folders, (2009-06-18-135821) to another drive and it wouldn't let me. It asked for a password at first, but then gave an error -50, couldn't do it. It made an empty 2009-06-18-135821 folder but the folder was not writable so it couldn't copy the contents.

I am curious though as to why you want to make a backup image to the same drive as the source material. If the drive goes bad, both backups are gone. It's just another partition, not another physical drive, so it's not really a backup of the backup.

HTH