An automatic USB scanner is a waste of resources. If malware is on the USB, it will be detected when it is accessed, starts or copied to harddisk. Depends on the file guard settings.
If you really want to scan USB drives, you could create a USB scan shortcut and place it on your desktop.
Open Emsisoft Anti-malware > Scan PC > Custom Scan.
Delete all drives that are shown, except the USB drive in top window (you need to have the USB attached at that moment)
Deselect all other settings below.
Save settings on your desktop with a name that makes sense to you. Doubleclick that file to scan the USB.