you'll have to edit smb.conf and more likely the two includes files it calls.
Before you start, please at minimum read the smb.conf directions on samba's website.
If you start fiddling around in these files before knowing what you're doing, you'll likely ruin samba. And I might suggest the old standby. BACKUP! your conf files.
Now that you've read the documentation… (you have haven't you? please don't shortchange yourself)
your smb.conf is located in /var/oxsemi/smb.conf
it has a few global directives and then calls two include files at the end. /var/oxsemi/shares.inc is your actual shares directory. /var/oxsemi/senvid.inc is a special share file which indicates the root folder for mionet as /shares/internal. (as a freebie hint you can change the username from mionet-username to your username or to www-data and have access to the full shares directory - beware though that it actually grants root level privileges.)
In order to make your thumbdrive mount you need to edit /var/oxsemi/shares.inc
create a new section (if one doesn't already exist) for your thumbdrive. in the following example you'll see the name thumbdrive in windows which will be your mount.
#extra portion of shares.inc for thumbdrive
[thumbdrive]
path=/shares/external/somethingorother
#
# you need to look up your actual path.
# the path may include the thumbdrive ID as well as a partition number
# for instance mine is
# /shares/external/SanDisk-U3-Cruzer-Micro/Partition-1
#
force user=www-data
guest ok=Yes
valid users=www-data #plus any other user names you want to include here
write list=www-data #plus other users as above
EDIT:
don't forget to effect your changes without a reboot just execute the following as root or sudo
/etc/init.d/samba.sh restart