I believe you need to deal with this disk a bit differently, especially in an Archos Recorder. Unfortunately I don't know much about it myself and can only point you to the
"BigDisk" wiki page which explains a lot about it. As I understand that one, it could mean that Rockbox can't write data away at all (or just in some parts?), though I find it curious that you seem to be able to listen to your music... flawlessly? This is not my area of expertise at all so I step back from guesses and hope someone else will chime in who knows better.
Another thing that keeps me wondering is that you don't mention how you start the recording. Maybe you just don't mention it though so I almost don't dare asking (don't take it personally, I just want to make sure) - but are you aware that you need to start the recording with a button press after entering the recording screen? As soon as you enter the screen, the (encoder here) chip is set to recording mode and you'll see the peakmeters move and you can monitor the sound but it doesn't actually record yet.