Assuming you're running RockBox...
- To make RB rescan the disk to figure out free space: menu -> info -> rockbox info hit play or navi and you should see a scanning message.
- Don't use the voice option in RB as that is still a source of some stability issues. To make sure, clear settings by holding REC while booting (but after pressing ON)
- During playback of long big files, don't try to stop it when it is still starting to load a file (known bug there)
- RB always writes to disk when recording so there's no 'save' to be done, just 'stop' to write the header. If it failed to do that, open the saved file as raw data and set the samplerate yourself.
- latest RB version adds improved recording framework and better abort on low battery (still have to check how that behaves while recording)
If you're having CRC errors, it could be that the connection with the disk daughter board has broken solderings. Or the disk may be bad.