The Archos units do not have disks that are readily replaceable. The firmware checks for certain specific information that is coded into the drive for the manufacturer specifically for Archos. There is a lot of discussion of this issue on this forum. Somebody has reported success with some fairly complicated hardware and will `fix' a disk to work with Archos, for a fee. Do a search
The general position is that nothing you can buy will work with any Archos unit without a great deal of work. My personal view is that you might get a better deal buying a new unit -- the prices have been knocked down by the release of newer models.