I recently purchased a 4XL and sold my two older HD units; I moved two of the TiVo expanders without incident to the 4XL and one of the series 3. The last unit would not pair with my 3rd Series 3. I had replaced the original WDC HD with a Hitachi and it had been functioning for years. I tried all the permutations for booting up and the TiVo would recognize the drive- even knew that it was a Hitachi, but would blow by the 3-Thumbs screen and return the message that the unit was not compatible. I spoke with WDC and the tech said SATA is SATA, shouldnt matter. But, I replaced the Hitachi with a 1TB WDC and had the same issue. TiVo says that it has to be a specially blessed WDC TiVo compatible drive although I cant find any part number that meets this criteria. Im at the point of diminishing returns- barring a solution from the forum.