There's another issue at play here.  Once you clone the drive, you absolutely must not boot the system with both drives connected.  You must remove the original drive from the system before booting the cloned drive the first time so it's signed as your master boot device.  Otherwise, the system will boot from the original drive and not the new one. It is not necessary to change the system to legacy mode.  You can if you wish, switch off secure boot after cloning, but you cannot have both drives connected when you first boot after imaging.  
Inspiron 15 (5570) running Windows 10, BIOS updated a week ago.  I've searched and all answers found don't appear to be quite the same issue. Installed a 1TB M2 NVMe, cloned the boot drive.  Both are GPT and have 5 partitions.  When I go into BIOS the boot order is being managed by Windows Boot Manager.  Secure Boot in enabled so I cannot change boot order.  I started to change things in BIOS - but when it got to the point where I had changed several items I backed out of there.  If someone can send me a link or describe the steps to change the boot order I would sure appreciate it.  Or if I need to supply more information please let me know.   Neale    
NVMe installed - How to change Boot order to get it to boot first