Simple question? Can I upgrade HDD from 0.5GB to 1, 2 or 3GB HDD?
Can I use any cheap 2TB or do I need to purchase a similar product?
See Things Every... (click) section 12 for a few pointers. Also af123's 2TB installation blog on the wiki HERE (click).
In summary: unless you feel particularly pioneering, 2TB is a practical limit and then installation will be straightforward using the current roll-out of Humax firmware (1.03.12) to format it through the menus. A drive optimised for AV use is recommended.
It may be possible to go larger, but then you will not be able to use the standard format option and the drive will have to be prepared by hand. Specifying larger sectors is a possible solution.