That´s not mandatory. The drives (USB or eSATA) can be selected in the USB backup plugin selection field even when they are not mounted. But they must be formatted with a valid file system (EXT4 is perfect ) and physically plugged-in.
And in order to start a USBbackup job they may not be mounted manually before. After creating a backup job the job should start the next time the device is connected.
Yes, thank you. Left the drive unmounted, ran the job and it's working.