How to write grub to external drive in MBR BIOS + EFI modes?

Question

linux

I install Ubuntu from a flash drive to an external MBR drive (MBR, not GPT). On the same computer, everything starts from this drive, but on an ancient netbook (C2D, 4Gb DDR2) - no.
I decided to reinstall grub, and I was surprised by the entry:

Installing for x86_64 -efi platform

And how to install it so that both the regular MBR BIOS bootloader and EFI are recorded?
For example, this is what Rufus does when burning an ISO to a flash drive.