Come on guys, it's been DAYS already! How come aliens still not in the picture???
He should at first assess his needs for backing up files. What kind of data for home use could possibly fill 20TB? Does he need to keep a backup of everything? Either way, you could always get a second (third?) NAS server and HDDs and setup automatic backups. A NAS with 2 bays will set you back 100$ or less. Add 2x 4TB for 400$. Better yet, build a custom file server for under 200$ with a nice case with plenty of room for disks. Be sure to choose a motherboard with 4 or more SATA ports and fill it with HDDs. FreeNAS is a great OS with many options. Five 4TB (=20TB) would cost under 1000$, so for 1200$ you have a nice long term solution with great flexibility.