How do I delete just the Time Machine files on my Drobo external filesystem? I have a folder on my Drobo called "HFS+ Private Directory Data" and it contains old Time Machine data. I cannot delete it and do not want to reformat my drobo. I have tried 'option' trashing and various Terminal hacks but to no avail. I am using Lion now. 

It's about 36 GB and I would love this space back. I'm running Lion on a 2006 MacPro and it's a four bay Drobo with 3 TB of 4 TB used. I use the Drobo for image files and a separate drive for Time Machine.

Any ideas?
Can't you delete it from the Time Machine interface?  Or is this a backup from a computer you no longer use?  If the latter then there shouldn't be much/any risk in just force deleting it.  If I remember right then you need to clear the immutable flag from the directory (or from all the files?)