I run a server with a couple of arrays, one main, one backup. The main is WD Reds as you would expect, but the backup array is a couple of 2TB Purple drives in a mirror. They seem perfectly suited to the role; they are optimised for writes and by the nature of backups they’re mostly written (incremental update nightly) and usually only read when something goes bang. This is a very similar pattern to their intended use of surveillance, only reviewing footage after a breakin.
Ignore the ATA Streaming stuff, they just have that command set enabled in their firmware, but won’t use it without getting those commands from the host. http://community.wd.com/t5/Desktop-Mobile-Drives/Wd-Purple-for-desktop-use/td-p/779395/page/2
They also have drive vibration protection for upto 8 bays, they are intellipower rpm like reds too, and have TLER enabled. Cheaper than reds though, as random reads/writes are pretty bad. Writing big single file images of volumes they are very good!