1) edit /etc/sysctl.conf and insert
kern.usermount=1

2) add the group that is to write to the USB drive to the operator
group in /etc/group

3) do #chmod g+w /dev/sd0i (sd0i is the mount point for the USB
thumb drive)

4) do mkdir /mediabsd (the user is bsd)

5) do chown bsd /mediabsd
this is based on the comment:

 /*
         * If the user is not root, ensure that they own the directory
         * onto which we are attempting to mount.
         */


    $ id
    uid=1000(tom) gid=1000(tom) groups=1000(tom), 0(wheel), 9(wsrc),
    10(users), 1001(openbsd), 1005(arches)

    $ sudo sysctl -w kern.usermount=1
    kern.usermount: 0 -> 1

    $ ls -l /mnt
    total 4
    drwxr-xr-x  2 root  wheel  512 Sep 25 21:18 tom/

    $ sudo vnconfig svnd0 /home/public/CD-R/Win98-Orig.iso

    $ mount -t cd9660 /dev/svnd0c /mnt/tom
    mount_cd9660: /dev/svnd0c on /mnt/tom: Operation not permitted

    $ sudo chown tom /mnt/tom

    $ mount -t cd9660 /dev/svnd0c /mnt/tom

    $ ls /mnt/tom
    AUTORUN.INF  DRIVERS/     SETUP.EXE    WIN98/
    BIN/         README.TXT   TOOLS/

    $ umount /mnt/tom


Hope this helps

Tom