Cannot free up space by deleting files

I have a My Book with 750 G.  I cannot do backups because my drive is showing full.  I manually deleted a bunch of files from the drive, but the drive is still showing full.  Aside from reformatting, how do I free up space.  I am running Win 7 64bit.

Keep erasing files, but clean your recycle bin as well and defrag the drive to make sure the remaining files are structured as they should.

One other thing delete the backup files with program that created them. Sometimes manually deleting them causes problems with data left in the backup software.

Joe