I ran this question by both weaknees and the folks at PTV Upgrade and they both shared that they are working on having upgrades available for this Tivo, but it is not possible to upgrade this Tivo at this time.
Seems that Tivo/Toshiba introduced some differences with this Tivo model that have not lended themselves to the presently available drive expansion utilities, thus proving this Tivo model to be a tough nut to crack.
And, in his e-mail response, weakness indicated to me that his team would likely be spending more time on developing an upgrade for this Tivo if there was more interest.
So, I'd recommend posting a reply to this thread to indicate your interest in the availability of SD-H400 drive upgrades (rather than badgering either of these groups with e-mails).

Seems that Tivo/Toshiba introduced some differences with this Tivo model that have not lended themselves to the presently available drive expansion utilities, thus proving this Tivo model to be a tough nut to crack.
And, in his e-mail response, weakness indicated to me that his team would likely be spending more time on developing an upgrade for this Tivo if there was more interest.
So, I'd recommend posting a reply to this thread to indicate your interest in the availability of SD-H400 drive upgrades (rather than badgering either of these groups with e-mails).