You *can* plug in an external drive, it's called a complete hardware duplicate of your array
You don't even need to duplicate the array hardware. 20 TB is still within the range of a RAID enclosure. Basically a box where you plug in drives, and it gloms them together into a RAID 0/1/5/10. You then plug it into your computer with eSATA or USB 3, and your computer sees it as a single drive up to 32 TB in size (8x4TB with current drives). Do your backup, then unplug it and turn it off until your next scheduled backup.
Incidentally, TFS is a perfect example of why RAID is not a backup. Yes RAID is kinda like storing two copies. But the "two copies" are linked both virtually and physically. If you accidentally delete one copy, the other gets deleted simultaneously. If you physically damage the array, you lose both copies.
RAID is not a backup. RAID is for redundancy. If your business will lose hundreds of dollars per hour or more while the file server is down, you put the file server on a RAID array so it will continue to operate even if there's a single (or double) drive failure. i.e. It only protects against your file server going offline. You still need a separate backup.