Just an FYI that the installer ignores your selection of Boot Loader, as it was intended for MBR/Legacy. The installer horribly, even when choosing "Something Else" and manually partitioning and creating an EFIin your install drive, will still install grub to the first EFI partition it finds, even if you are following a guide like this to avoid wiping out the M........
mdadm --manage /dev/md1 --add /dev/sdb1
mdadm: metadata format 00.90 unknown, ignored.
mdadm: metadata format 00.90 unknown, ignored.
mdadm: metadata format 00.90 unknown, ignored.
mdadm: /dev/sdb1 not large enough to join array
md1's first primary member /dev/sda3 has 57394 cylinders while the /dev/sdb1 has 57393 (1 less cylinder) which is why it won't work.
fdisk -l /dev/sda3
Disk /dev/sda3: 47........