I recently purchased a TiVo (model # TCD540040) that included an InstantCake upgrade CD. I used this CD to prepare a 500 GB Western Digital (model # WD5000AVJB) HDD for this TiVo. After the upgrade, the recording capacity for the basic setting was 571 hours. I then did the upgrade on a PC with linux installed and used MFS Toools 2.0. I ended up with the same recording capacity of 571 hours.

I see that weaknees.com has a 500 GB kit available for the TCD540040 model TiVo that shows an expected upgrade capacity of 626 hours. Why am not able to get 626 hours with this HDD? Is the lower recording capacity the result of the HDD not being identical (same make/model) to that in the upgrade kit? Any help would be greatly appreciated. Thanks.