External HDDs and SSDs are all well and good, but they're usually made with casual use in mind, not for long-term backup tasks. Besides, regular SATA drives usually work best for such things.
That is why companies like Icy Dock are launching things like the new ToughArmor MB991SK-B 2.5-inch SATA Mobile Rack solution and FlexCage MB975SP-B 5-in-3 Tray-less SATA 6Gbps HDD Cage.
The former is a small form factor made for home servers, industrial PCs, SMB SFF servers, and mobile DVR systems. Single drives are kept safe and secure within (1 x 7mm to 15mm 2.5-inch SATA drives/ SSDs in a 3.5-inch floppy drive bay).
The latter is a five-drive case for unRAID, Windows home servers, gaming / media / server computer cases, etc. It has plug & play Tray-less design and a replaceable 800 fan.