After clonin the disk you should resize the partitions using gparted - this should recreate the GPT block at the end of the partition. Without this you'll most likly have problems booting. Also make sure your old drive is not attached to the computer when booting.
Nik
This has all been done. I can boot from the MX Linux install stick, but I can't install/reinstall Grub.
I know I can install MX directly, but that would mean reinstalling a lot of stuff, so I'll probably stay with the original 256GB SSD for now.
I've reinstalled Grub on several occasions, but it was mostly grub mbr. I don't know if this is a problem with grub-efi or if my install is somehow corrupted. This error message makes no sense, as there is no such file available and the grub install was done by the system...
Thierry