As there are a number of different unit types there are a number of links provided here:
If you wish to upgrade the Amstrad HD Box, please use this guide. How to upgrade the Hard Drive of your Amstrad HD box
If you wish to upgrade the Thomson HD Box, please use this guide. How to upgrade the Hard Drive in your Thomson HD Box.
If you wish to upgrade the Pace HD Box, please use this guide. How to upgrade the Hard Drive in your Pace HD Box
If you wish to upgrade the Samsung HD Box, please use this guide. How to upgrade the Hard Drive in your Samsung HD Box
I recommend you use a Hitachi DeskStar 1TB Hard Drive SATAII 7200rpm 16MB Cache