EX2 Ultra can't find USB device to back up

Trying to create a backup to my EX2 Ultra from a USB drive attached to my MacBook Pro, but the dashboard says “There are no USB devices found” (and therefore there are no choices available in the backup job “Select Source Folder” window).

I feel like I’ve done this before successfully with drives attached to my Mac, but do I have to attach the USB drive directly to the EX2?