The string containing the file name of the next directory member or an empty string if all file names have already been returned. The method also affects the state of fd.laststatus. The following status codes are possible:
0- PL_FD_STATUS_OK: Completed successfully.
1- PL_FD_STATUS_FAIL : Physical flash memory failure (fatal).
2- PL_FD_STATUS_CHECKSUM_ERR: Checksum error has been detected in one of the disk sectors (fatal).
3- PL_FD_STATUS_FORMAT_ERR: Disk formatting error has been detected (fatal).
8- PL_FD_STATUS_NOT_READY: The disk is not mounted.