There is a very short list of WD drive model numbers (of the actual raw drive itself) which any TiVo newer than the original USA Series 3 (the one with the clock display) will have and against which it will check any drive attached to the eSATA port. If the drive inside the external enclosure is not one of those few models, the TiVo will refuse to work with it. If the drive inside your external has truly failed (as opposed to just the enclosure or the power supply for the enclosure failing), then unfortunately, because of the way TiVo divides each recording between the internal and external drives once you hook one up, you've lost those recordings. If this is the case, you need to instruct the TiVo to divorce the external drive. After that is done, I suggest you use the Australian version of WinMFS to copy the internal to a 2TB (success seems to be being had in great numbers with either the WD20EURS or its successor the WD20EURX), and then, as a separate step, expand into the remaining space. Having two drives doubles the chances of failure.