I didn't create the MBR. I had copied my 500 GB hdd to the NVME using
"dd" because it contains Windoze 10, for which I have neither
installation media nor product keys. Then I used gparted to expand
/home, and create the EFI partition. I only use Windoze about once per
year, but I didn't want to blow it (and my home directory) away by
converting to GPT.
Interesting. Does that mean that you switch to BIOS booting when you
run Windows? I've been led to believe that, while all four combinations
of MBR/GPT format with BIOS/UEFI booting are workable, Windows does
not allow UEFI booting with MBR disks. (Strictly, I've read that
Windows won't install onto such a combination, but not that it can't
boot if it somehow finds itself in that situation.)
I haven't tried booting Windoze since I copied to the NVME. I recently read that (at least in Dell Latitude E5470) one can only boot from NVME using UEFI. I noticed that OS probe didn't find Windoze. I have a vagiue recollection that UEFI grub can't boot MBR Wiindoze, and vice veraa, so maybe my Windoze is now useless anyway.
Maybe I need to buy a 1/2 TB thumb drive, back up my /home, buy a WIndoze 11 installation DVD and product key, and start over with the NVME empty, making it GPT instead of MBR.