Regardless of drive purchased, the best way to get an external is build it yourself, as the retail packaged ones of today
does not have a SATA interface if disassembled. Meaning if a retail one model, if dropped & something like a solder joint breaks, the drive is useless & data lost. One would have to pay far more than the drive is worth to recover any data & the charge will be higher than that of a traditional SATA based model.
Purchase any 3.5" recommended brand (based on reviews) 1 to 2TB HDD on pomo & a 3.5" USB 3.0 powered enclosure. Of if you prefer smaller, go with a 2.5" drive & enclosure, it'll be self powered from the USB port. You'll likely be limited to mainly 1TB tops with the 2.5" enclosures (& you need capacity). The cost of enclosure will also be less with 2.5", although with either choice, make sure the body is metal, as plastic ones will cause the drive to overheat during long backups. The metal helps to get of heat faster.
I don't recommend a SSD for backup, unless there's an eSATA port on the computer (& enclosure has an eSATA port to connect), because w/out a SATA connection, there'll be no TRIM or garbage collection. Both are needed for performance & longevity of the drive. Yes I know SSD's are at all time lows in pricing, however many of these are of low quality (some w/out physical DRAM) & won't last long as a backup drive. Am not sure why some of the OEM's are introducing these for portable use due to the lack of TRIM & GC being available. Maybe their goal is to sell more drives due to short lifespan.
I still have my first two SSD's in usage since 2012 (128GB Crucial m4 & 180GB Intel 330), installed in notebooks & running fine. If these were in enclosures, would be dead today. The Crucial m4 still has 98% lifespan & Intel 330 100%.
Here's a couple of recommendations.
https://www.amazon.com/Seagate-Barr...enclosure&qid=1566121567&s=electronics&sr=1-9
Fast drive, great price for 2TB!
https://www.amazon.com/dp/B00UAA4J6..., Aluminum USB 3.0 Sata HDD Enclosure, FE3001
That's the type of USB 3.0 port which is best, there's another slim type cable that gives trouble. Any that's shaped like this is good.
Good Luck!
Cat