Spike2k5′s previously mentioned tool, WinMFS, is now available for download as an alpha release. WinMFS allows you to image new drives to install in a TiVo under Windows, unlike the older MFSTools which required booting from a Linux boot CD. WinMFS is also the first publicly available tool which can image drives for the new TiVo HD. Since this is an Alpha release, it is not recommended for the timid. Definitely read the WinMFS Guide before signing up to download it.
In related news, DVRupgrade tells me that they’re now comfortable using WinMFS to begin imaging drives for expanded TiVo HD units. They have a shipment of TiVo HD units inbound, and they’ll begin installing the drives in the units and shipping them out to those who have pre-ordered ASAP. Their first priority will be filling the pre-orders for complete units, then they’ll begin selling self-install upgrade kits.