[ubuntu] I installed the boot loader to the wrong device but partially like the results

I had a PC with
Wndows on /dev/sda
Xubuntu 18.04 Desktop on /dev/sdb

I selected which to boot by switching between UEFI and Legacy boot.

I did not manage EFI boot becuase i could not get the Live 18.04 Xubuntu to EFI boot (and I wanted to leave the Windows disk untouched).

So just ran with that and used the boot mode to select the operating system.


I installed Ubuntu 20 Desktop to a memory Stick while in EFI mode.

and it booted.

What is more when booting from the memory stick in EFI mode: GRUB offered me the option Boot:
1) Ubuntu - 20 from the memory stick
2) Windows - from /dev/sda
3) Xubuntu 18.04 from /dev/sdb

It would boot all three operating system successfully via the GRUB menu on the Memory stick.

So I did a silly thing I reinstalled Ubuntu 20 to the stick BUT put the boot loader on /dev/sdb


So now I can EFI boot the second disk Xubuntu 18.04 form /dev/sdb but I must have the Memory stick present or the boot fails to the GRUB command line.

Can you help me get the it to boot EFI mode without the Memory stick present.

I want to keep the Xubuntu 18.04 on /dev/sdb and EFI boot.
I don't care about the Ubuntu 20 on the memory stick.

Thanks