Putting in a new drive is easy.
I used a WD Black but I'd think a Red or Purple would be fine.
Unable to post guide publically as it's not an approved modification.
I used Samsung Spinpoint ST2000LM003 successfully in both my boxes. I don't think that they are available new anymore, but ISTR that the main spec to look for is low power as some drives don't start up properly. Don't bother with a 7200rpm drive, a 5400rpm, (such as I used), is fine.
Does this invalidate the warranty at all?
Without taking my box part it looks like a 2.5in SATA drive, is that correct?
Warranty: probably. 2.5" drive: yes.