The next step would be to debug the function fatfs_write(), add logs, debug the function to find out exactly where -5 (-EIO, I/O error) is returned. If you figure out for example that f_write() returns -5, the keep debugging the f_write() function and figure out where that fails.
However, I noticed now that you have double posted this. Please continue the discussion in fs: file write error (-5). I'm closing this one.
