The operation can’t be completed because backup items can’t be modified.

I have an older LaCie external hard drive that is almost full with a lot of files from backups that I want to get rid of, but when I try to delete anything, I get the "The operation can’t be completed because backup items can’t be modified." message. If I try to delete anything through the Adobe Bridges window, I get the "You don't have sufficient permissions" message.

I have searched the forums here, but have not found anything that resolves my issue. The LaCie is Mac formatted, and I have checked "Read & write" for administrator permissions, still no go. Any solutions to my dilemma are greatly appreciated.