Transmission: how to configure usb Port?

I tried thousands of time to map a usb port address on mycloud, but failed.

Everybody knows that when we use Transmission on mycloud, Transmission uses a folder “Transmission” under “Public” folder. The default Transmission download folder is:

/mnt/HD/HD_a2/Public/Transmission

I tried to use a cased external USB drive for Transmission, but I don’t know how to configure the location. I tried the whole internet and couldn’t find a solution.

Anyone who could help with this simple question?