
Backup sda1
and reformat it as FAT32
# cd /mnt
# mkdir sda1 tmp
# mount /dev/sda1 /mnt/sda1
# rsync -ravt /mnt/sda1/ /mnt/tmp/
# umount /mnt/sda1
# mkfs.vfat -F32 -s 2 /dev/sda1
# mount /dev/sda1 /mnt/sda1
# rsync -ravt /mnt/tmp/ /mnt/sda1/

Set to UEFI boot.