If that device /dev/sdb/ is the first in your firmware boot order then MBR would be in that device’s MBR.
Assuming the above is correct - this would be the command to sync the disk grub with the new version
grub-install --target=i386-pc --recheck /dev/sdb
Also see Install-grub: a new way to keep your EFI/MBR in-sync with grub package