Here are some notes about doing simple stuff via SSH in FreeNAS, my memory isn't quite what it used to be.
First, find out the path to your USB drive by executing dmesg and reivewing the output.
$ dmesg
ugen1.2: <Seagate> at usbus1
umass1: <Seagate GoFlex Desk, class 0/0, rev 2.10/1.00, addr 2> on usbus1
da1 at umass-sim1 bus 1 scbus7 target 0 lun 0
da1: <Seagate GoFlex Desk 0D18> Fixed Direct Access SCSI-5 device
da1: 40.000MB/s transfers
da1: 1907729MB (3907029167 512 byte sectors: 255H 63S/T 243201C)
From the above, you can see that the USB drive is located at /dev/da1, now look for the paritions on that drive using:
$ ls -l /dev/da1*
crw-r----- 1 root operator 0, 143 Jun 30 05:01 /dev/da1
crw-r----- 1 root operator 0, 144 Jun 30 05:01 /dev/da1s1
We know know that the first partition is located at /dev/da1s1, next we can try to mount that drive
$ mkdir usb-drive
$ sudo su
$ mount_msdosfs /dev/da1s1 usb-drive
If you have a USB drive is a large FAT partition, you will need to add the '-o large option' to the command.
$ mount_msdosfs -o large /dev/da1s1 usb-drive