In case you are looking solution for any other freenas/bsd version than 9.1 check this link: FreeNas: Realtek 8111F and “re0: Watchdog Timeout Error” universal solution
In the world of operating system that support most of available hardware sometimes a little hiccup occurs that make you a headache …
In case of a Realtek 8111F Network adapter or to be more specific RealTek 8168/8111 B/C/CP/D/DP/E/F/G there is a driver bug that lockup entire network adapter when there is big data transfer that take long time.
You could add extra network adapter (as the Realtek is usuly build-in) but you can also upgrade driver.
Driver for FreeNas 9.10 x64 – if_re.ko.v190.zip
cp if_re.ko /boot/kernel/if_re.ko
chmod 555 /boot/kernel/if_re.ko
#Settings > Tunable > Add:
variable = if_re_load
value = YES
type = Loader
enabled = true