I'm afraid even if the drives were the same physical shape and connection system, they are linked to the operating system by individual serial numbers, so a swap would not work even if it was physically possible.
I have bought replacement drives for locked Archos products from these peoplehttp://s289462194.e-shop.info/shop/page ... hop_param=
and can confirm they do work. You will have to supply the serial number of the bust 504 drive for them to replicate a substitute. So far as I know these are the only people offering to replicate the drives.