Help-Reboot and Select proper Boot device -after restoring startup disk

[NewbieEditor]

I recently did an OS upgrade from El Capitan to High Sierra but due to unresolved NVIDIA driver problems, I decided to go back to my previous El Capitan install.
-Before the HS update, I had used CCCloner to make a backup of my El Capitan internal SSD startdisk on an external HDD.
-I rebooted from the external El Capitan backup, erased the internal SSD via Disk Utility and restored it via CCCloner.
The internal SSD now appeared as running on OS X El Capitan and looked identical to how it was before the HS upgrade.
-I selected the restored internal SSD as startup disk and restarted the computer.

-I now get this message "Reboot and Select proper Boot device" :
When I press F12, the BIOS detects both the internal SSD and the external backup HDD. Whether I select one or the other, nothing works.
I don't understand why, because I was able to successfully boot from the external backup HDD before restoring the internal SSD from the HDD.
Boot Option1 is set on the internal SSD.
"Storage Boot Option Control" and "Other PCI Device ROM Priority" set on UEFI Only.

Thank you for your help !!

INTEL Core i7 6700K 4Ghz
GIGABYTE Z170 UD5
GIGABYTE GEFORCE GTX 980 4Go VRAM
G.Skill Ripjaws DDR4 32 Go RAM
SSD 250 Go/ for OS install and Applications
Internal HD for Projects and files

External HD for OS install and Applications backup
OS X El Capitan 10.11.6 (aborted update to 10.13.6 then attempted downgrade to 10.11.6)

 

[NewbieEditor]

Anyone? Any ideas?
I'm stuck with "Reboot and Select proper Boot device" and starting to lose faith