When mounting an (external) NTFS disk in FreeBSD, especially in TrueNAS right here, with write permissions (777 actually) you would do the following, which can also be found in the FreeBSD manual

# list currently attachded disks
geom disk list
# load fusefs
kldload fusefs
# or install, if not installed - then load like shown above
pkg install fusefs-ntfs
# make your mount directory for your disk, e.g.
mkdir /mnt/usb
# show the partitions on the disk, decide which one to mount, e.g. the first, 'da0s1'
gpart show da0
# ...and actually mount it, use ntfs-3g instead of 'mount'
ntfs-3g /dev/da0s1 /mnt/usb/
# and/or add an fstab entry for automagic mounting after boot
# /dev/da0s1  /mnt/usb	ntfs mountprog=/usr/local/bin/ntfs-3g,noauto,rw  0 0
# and unmount if you are done
umount /mnt/usb