Backblaze has posted another report on the reliability of their hard drives https://www.backblaze.com/blog/hard-drive-reliability-q4-2015/. As pointed out in the Ars Technica analysis http://arstechnica.com/information-technology/2016/02/hgst-hard-disks-still-super-reliable-seagates-have-greatly-improved/, HGST is owned by WD, so WD is simultaneously responsible for the most and least reliable brands of drives in the report.

An overall statistic of 6% failure rate per year seems a reasonable conclusion. Also it seems like 3-4TB is still the sweet spot.