Probably you have a bad/corrupted flash drive.
Do you have a backup of your freenas configuration?
Can you install freenas on another flash drive and restore the configuration?
You can install a different version of FreeNAS. However
you cannot count on being able to install an older version, since with the older version your ZFS pool/volume might not be accessible;
when installing a newer FreeNAS (generally, a recommended course of action) you will be likely getting a newer version of samba, so some details of CIFS setup could be different.