For those who may be tempted to take WK using the WD Purple as a reason to buy one for DIY, you'd be better off with a Red. They also go to 6TB and are AV drives (support the ATA AV streaming command set extensions), not that it necessarily matters, since TiVo doesn't use them).
Purple rated workload: 60TB/yr
Red rated workload: 120-140TB/yr
Both drives are for NAS (thus have TLER/ERC, which can be a detriment on a non-RAID controller). But, the Purple is designed specifically for motion-activated surveillance, not true 24x7 (unless you count idle time as 24x7). Since TiVo can't do RAID, you can't just add more drives, to meet the TB/yr workload, as WD expects, if you exceed it with one drive. The rating is per drive.
If you can wait to buy, sales on the Red NAS are frequent, often coming very close to the regular price of a Purple. Obviously, WK is going to go with what they can always get for the lowest cost, to increase their margins, and only cares that the drive last as long as the WK warranty.
I've done the math, and 4-tuner TiVos can easily exceed 60TB/yr, unless you don't record in HD. Don't forget that all tuners are always buffering, which still counts towards this, and 6-tuner TiVos insure there's no (logical) way to not exceed the workload rating.
For telemark's sake, and his thread, I won't go into details about what I found out about, through experience, with TLER/ERC drives, used in TiVos (I use WD Red). It seems to take a "perfect storm" of conditions to bite you, which is why I'm trying to just keep it as a footnote to everything else (and have not stopped using Red drives).
ETA: WD still only considers the AV-GP as the proper drive for non-RAID DVR/PVR use. You will not be able to get assistance from them, if you try to, for other drives. While their warranty terms clearly state the warranty is only valid for intended drives for intended use, getting warranty RMAs for other drives has not yet been reported as an issue (which they could start enforcing, and change that, at any time).