Ministation Stealth full - how do I get rid of old backups?

[Rigel]

I'm using an HD-PCT500U2/B-US Ministation Stealth for laptop backups.  A few days ago, it informed me it was full.  I'd like to clear off the oldest backups to make room for new ones, but there doesn't seem to be any functionality to do so.  I also cannot simply delete the folders with the oldest backups because my Win 7 system can't seem to trash them - it deletes all the folder's contents, but can't delete the folders.

Is there a trick to getting rid of old backups so I can continue to use the hardware?  Or am I stuck with the brute force solution: format the drive and start using it again from scratch?