If reliability is the most important factor and you don't mind 3.5" drives, take a look at Backblaze's hard drive comparison
. The HGST drives have the lowest failure rates. Buy your own case if you can't find one ready-cased.
If you want extreme portability, then take a look at Seagate's Backup Plus Slim
series. I have two 2TB drives that I rotate just in case one fails, and I shall probably add another. WD do similar drives. I wouldn't use just one of these drives for backups since they are really pushing hard drive technology. You only have to read the sob-stories on Amazon to realise how unwise this is.
Edited by rumster (Fri 13-May-16 17:43:23)