How do I delete a TM backup?

Here's the situation. I have 4 HDDs on my MP. HD1 is a 1.5TB drive that is backed up by HD4 (my TM drive), also a 1.5TB drive. HDs 2 and 3 were both 500GB drives. In my TM preferences I had excluded HD2 and HD3 from being backed up. I had filled up HD3 so I got another 1.5TB drive, pulled HD2 out and added the new drive and then proceeded to copy all the files off the drive that was full. Once completed I then swapped HD2 and HD3.

In the process, I forgot to add the new 1.5TB (now called HD5) to the do not back up list in TM. So, since this morning my TM drive has been backing up HD5 which has about 430GB of data that I don't want backed up using the TM drive. When I double click the TM drive icon on my desktop I can navigate to today's backup and see where it says HD5 but it won't let me delete the folder or the files. When I try and move the folder to trash it says, "The operation can't be completed because backup items can't be modified."

I've since added HD5 to my "Don't back up list" but how do I delete all this stuff on the TM drive? I want to free up this 430GB space.