What you need is:
When all prerequisites are met, all that has to be done is to connect the USB mass storage device to the USB port of the router. FreeWRT provides a hotplugging script which mounts all connected partitions under /mnt/discN_Y while N is the index of the connected disk (i.e. starting with 0) and Y is the number of the partition on the disk (i.e. starting with 1).