Erased files = no change in size

I have a MacBook v. 10.6.8 and would like to back it up using my WD external hard drive.  I bought the drive years ago and at the time had a PC.  I’ve just deleted all large capacity files from it so I can use it to back up my Mac.  Even after erasing all of the files, the WD drive shows no additional space.  What can I do to fix the problem?  Any ideas???  

If you only wish to use this on a Mac just reformat it and start clean.