I'm afraid it's not that simple. The specific hard drive is tagged to the device so that you need to firstly get a drive of the same size as original but then also change the drive so that it mimics your previous one in terms of serial number etc. This last part requires specialist equipment not normally available as far as I'm aware.
I had my 500gb drive replaced twice by an aftermarket firm in the US that specifically does this (the first replacement died after a few months but they replaced with no issue as these come with a 3 year warranty which is better than archos.
Details per below if you need to go down that path: http://archosharddrives.com/shop/page/1 ... hop_param=
BTW - apparently if you use the SDK developer software you may be able to get around this but I have not used and cant comment further on this aspect.