I have a DRX-890, not a Thomson, and I managed to fit an external disk enclosure as described here: http://www.skyuser.co.uk/forum/sky-hd-s ... hdd-6.html
I had previously bought a dual disk caddy but decided to use a proper enclosure to protect the disk from physical damage. However, the critical issue seems to be the Esata cable length as I could get no consistent results if it is >50cm (on top of the length of the existing cable, with a connector)- whether that is also true for the Thomson I can't say, but I suspect that it is!
The other critical issue is the the power to the external disk MUST be switched ON a few seconds before the SkyHD box is powered up from the mains - otherwise the disk does not settle to correct operation before the SkyHD box searches for it, and so you can't access any programmes stored on the disk.
PS: no change since the linked post as I have been too busy with other hobbies, DIY and work - although the latter may change as I have just been made redundant!!