Just so I understand, you are using the Image Only version of the backup to the USB 3.0 drive? You are imaging the System Partition and the C: partition?
Does your external drive work normally otherwise, and is found for a possible backup location?
Do you have any library folders that are not on one of the imaged partitions?
FYI, when you do get it to work, be advised if you try to restore an image using a USB 3.0 device in a USB 3.0 only port, it probably not work. You should be able to move it to a backward compatible port.