Use something like GPartEd to resize your current partition and create a new one. If you are wise you will take copies of any important files before doing this.
TBH a partition on the same disk is not the best place for backup files. If the disk fails, so have your backups. (Yes, hard disks do fail - having just replaced one I can testify to that.) Another internal drive or, better, a cheap external hard drive is a more reliable solution.