Defragmentation Issue on Norton Utilities

On Windows 7, my backup System Drive (1 Tb) only has about 29 GB of space left.  Thus, when I attempt to defragment it, I get a message that defragmentation is not possible.  But the drive consists of about 1,000 "pgbleach.xxx" files, each named in sequence.  I don't think I can remove any of these files without corrupting the drive and then not being able to use my computer at all.  How can I downsize the Backup System Drive so that I can defragment and optimize it?