A few Inkbunny staff host off-site backups, primarily to ensure continuity of service in the unlikely event of a disaster at our datacenter. My own has been on a virtual machine, running from a dedicated 4TB hard disk hosted on my tower. (Fun fact: it cost $20 less to "shell" a backup device than to buy a bare disk.)
I could have built my own backup box, but it's hard to get equivalent build quality and capabilities at ~$156 including rebate. There are other nice things about it - nine separate temperature monitors, plus those on the CPU and hard disk; a single large, low-powered fan; internal slots for a USB 2.0 stick and microSD - neither super-fast, but fast enough to be useful, and if I need more there's PCI-e 16x as well.
Plus, it's cute! It fits under my desk, and uses an open VGA input … though I've yet get Linux to show the full 1280x1024. The server's official name is acedia, but I think of it as a miniature Companion Cube. Efficient one, too - at ~5%, the CPU's probably not taking more than 15W, and after that it's 7W per disk.
By the time it needs more than one, I expect 3.5" 4TB disks to be relatively cheap. There's room for four in total - plus an SSD up top. If that's not enough, I can slip four more 2.5" drives in - an inventive soul made a bracket to mount them. With 2x1Gbps ports, there's plenty of bandwidth. Good to have options!