I suppose SSD's are still too expensive to use in these types of receivers. Maybe in the H4.
While some manufacturers of SSD's have claims of multiple read/write cycles that suggest no early failure compared to spinning HDD's, I don't believe them, yet. I can't remember where, but I came across two different articles of tech journalists who used the newest SSD's, and still had early failure, due to how SSD's work (the electrical charge). The very nature of DVR's that write, then write over at some point or re-write over and over is just asking for trouble today in such devices like DVR's. There are a few on the TiVo Community Forums who say their SSD's have done well, so far, in their TiVo's, but how much of the recordings are they saving and preventing re-writes?
I have no doubt that soon we will have SSD's that may even out perform spinning HDD's in frequent write, re-write devices and at good retail prices, but I don't think we are there yet, and so far, no evidence that the best ones with the biggest claims are doing so, but rather evidence to the contrary. Considering cost and apocryphal claims, SSD's are still best for functions that don't require much re-writes, like a boot-up disk or the like. Time will tell.