Backup hard drive

I'm having another Windows 7 Home Premium SP1 problem. My desktop
computer has three Western Digital SCSI hard drives. Drive C: is used
for my programs. Drive D: is used for my client software working
files. Drive F: is used for a back-up.

Each of these hard drives is 465 GB and, except for the back-up drive,
have a lot of free space. The back-up drive shows 465 GB used with
33.1 MB free. This is the problem. There isn't enough room for the
automatic back-up process to do its job and this causes my computer to
stall out while the drive light indicates a continual activity.

I looked at the contents of the back-up drive in Windows Explorer and
the total of all the files/folders doesn't come anywhere near the 465
GB shown as used space. I'm thinking this drive has multiple copies of
some of these backed-up files. If this is right, I would like to
delete those older copies then do a disc clean-up and see if this
would resolve the problem. How do I find and delete those older
copies, if they do exist?

Thanks, Gordon