it is a well known problem when using windows as they (sometimes) fail to recalculate the free disk space.... And RockBox does not automatically do that - it would only consume more battery.
The strange thing is that this does not always happen....
Oh and the bug where RockBox fails to write the recording properly and causes a minor disk corruption (no data lost) is not completely fixed, I'm still hunting the bug whenever I manage to find some time... Raw import and chkdsk /f do the trick