Visted my local Staples today, picked up two 2tb Western Digital My Book Essential external drives for $71 each. According to what I can find online they have They have WD green 64mb cache drives in them, I believe the drive model is WD20EARS but I haven't torn mine apart yet as I'm using them for their intended purpose as external drives to do PC data backup.
$71 appears to good esp considering you are getting a USB 3.0 external drive interface, note that the shelf price was higher and it wasn't until they pulled the drive from the back room, brought to the cash register and scanned it did it ring up at $71. This might be a fluke as I can't find this listed on Staples website for this cheap. I wasn't using any discount card or anything, I was tempted to buy the 3tb external but 2tb was all I need right now.
Btw they also have 4tb drives but they may not have WD drives in them, the 3tb versions are reported to have WD drives.
Note, the external drive has WD encryption, it may need to be low level formatted before using in your Tivo esp if you use it externally first, per online reports it appears no one can access data after installing it in their normal desktop PC using SATA.