In order to be able to read and write to USB devices, users need to be members of the operator group:
# pw groupmod operator -m jerry
Then, add the following to /etc/devfs.rules (create it if it does not exist yet):
[system=10]
add path 'usb/*' mode 0660 group operator
To load these new rules, add the following to /etc/rc.conf:
devfs_system_ruleset="system"
Then, restart devfs:
# /etc/rc.d/devfs restart