You need to boot off the USB drive, and you should be able to access the drive at that point.
Unfortunately the boot from the USB drive takes me to the Acer eRecovery which has no option to nominate system backup files. It seems this full system backup process has been designed for burning a set of DVD's to restore from. This ultrabook has no optical drive which is why I created the system backup onto the external hard drive. I expect that there is some simple trick, such as copying the last smaller system backup file to the USB and/or adding a command on the booy USB. Any other suggestion?