Disk cloning using SupportAssist

I cloned (or so I thought) internal 256 GB ssd to a 1 TB ssd that is in a USB enclosure. This is the only way I can do it because XPS 8930 has only one M.2 ssd slot on the system board.

I then tried to boot from the target ssd and I got an error 0x7B - Inaccesible_Boot_Device. As can be seen from the snips, the crucial item, 'Boot' is not on the target ssd, nor is it marked as bootable in This PC.

So, there is no point in swapping the disks.

I would like to hear any advice, views etc about this.