My Passpport deleting files during backup?

My disk space was low so I bought this 1TB My Passport.  What I want to do is backup my files to the Passport and delete them from my hard drive.  I have mostly photos that take up a lot of space.  So after it completed the backup I started deleting folder and files.  I noticed at the time the file count in the WD Smartware began dropping on the Passport as well as my hard drive.

Is it synchronizing the file list and removing files from the Passort that I am deleting from my hard drive?  THat’s what it looks like it may be doing.  Should I be unplugging my USB while I delete the files and if I plug the Passport back up will it see there’s no files to backup/syncronize and remove them from the Passport also?

I don’t think it was designed to function like this but it looks strange with the file list number decreasing like that.