backups for computer using iPhotos?

I have just converted from Aperture to iPhotos. My question is about backups. I have an iMAC (OS10.11 with a 3 terabyte hard drive and the external drive is 2 terabytes with 1.34 TB available. However, I am also using space on the drive as vaults for Aperture. Shall I go ahead and reformat in order to have the whole drive for back ups?

Thanks