On a Raspberry Pi 3 I have successfully enabled hardware boot from USB mass storage device, so I am able to write a Raspbian image on a USB hard disk and boot the pi from it. 

Now, I have an already existing image on a microSD that I want to copy on the USB hard disk. Unfortunately, the system does not boot. So, there is something I have to do to boot an image copied from a microSD.

Any ideas?