I'm sure I read somewhere that a 5TB worked as an external drive, I can't confirm though.
The fvp4000t is very limited as far as file manipulation goes. Folders cannot be created so everything stays at the top level in "Recordings" which will become more of a problem with time.
Files can be copied across from usb and land in a different area under "Media" on the internal hard drive, folders will be copied across, but cannot be created so any manipulation needs to be carried out before copying over.
Files can be copied over via network and land in yet another area under "Media" on the internal hard drive which is a samba share and as such all files/folders can be manipulated in this area.
In normal circumstances only SD recordings from the hdrFoxT2 will be visible on the fvp4000t regardless of them being available via usb or network, the associated 'info' is lost though.
My advice, same as others here, install custom firmware on the hdrFoxT2 if you haven't done so already as this will allow a whole host of options.
The fvp4000t although having better hardware is still lacking as far as software goes and shouldn't be seen as a media server. The UI (user interface) is limited and frustrating and it's not just a case of getting used to it. If it weren't for the online content, catch-up etc. a 2nd hdrFoxT2 would make more sense.
Some users are happy with 500GB and use a pvr mainly for time shifting. Seeing as you're already using large external drives I'd recommend the 2TB fvp4000t model over the 1TB.
With custom firmware you can fit a larger than 2TB drive in the hdrFoxT2, I can't remmeber if >2TB internal is possible with the fvp4000t. I can imagine bbc changing/updating iplayer yet again over the next year or two so the fvp4000t is the way forward as far as online content goes, but don't expect too much from it.