Just noticed your post as I finished.Coming in late on this, I pulled a Western Digital WD10EURX drive out of a Sky HD box recently and it's still available - Scan £54.49. for example.
Although a lot of HDD's work fine the general opinion is that slower spin speeds are quieter (but by how much is hard to confirm), especially in different H/W. The Humax was developed after the Digitalstream by the same company and they improved the soundproofing/damping.
I am not sure how old the WD10EURX design is, but it was not mentioned in WD's email but is useful info.
From my recent research I think most AV HDD's will work in the box, I had an ordinary PC HDD in my old DS for a year but that's not really recommended!