How to clear files off my WD Elements External Hard Drive (500Gb)

I am backing up approximately 50Gb of files daily (with 2 copies of files setup in settings) to a 500Gb Elements Hard Drive.

The hard drive is now telling me that it won’t back as it is full?

I cannot understand why the drive is full ?

Also I would like to find out how to erase the files on the drive, without reformatting the whole drive (so as not to loose the WD software on the drive) ?

Can anyone suggest to me what to do or what i am doing wrong?

What backup software are you using?