On my jtag i have a 1tb laptop hdd in the m$ caddy, and 2 external hdd's via usb.
If i try to connect another usb storage device iether hdd or flash it does not recognise in xexmenu or freestyle's file manager, this is due to the microsoft 2 device limit, so the max number of external usb based storage is 2x but any size as the 16gb cap does not aplly to jtags so you can have 2x 2tb hdd's hooked up and will work just fine.
Hope that answers your question
