There are many Qs how to setup new samba in my 15200 builds (big and std-usb-ftp)
This is how it works for me - please try - feedback welcome
on NAS page, enable Samba, set server string to e.g. Samba Server, set the Workgroup to your e.g. WORKGROUP, set Public share = Enable, Read Only = Disable, Custom config = Disable. Apply.
Samba is currently reading /jffs, so please select mount point for USB to /jffs (if you mounted your USB to /mnt, you simply do mount --bind /mnt /jffs )
Now you shoud be able to see files remotly