Comment 38 for bug 1396379

I think I have just been hit by that bug too: fresh install of (X)Ubuntu 20.04 on a new computer with a new SSD, which also happened to have an old HDD with an existing EFI boot paritition. I chose the "expert" option for the partitioning dialog in install and explicitly specified the new SSD as the intended boot device. The install used the existing EFI on the old HDD instead.

Recovering was not easy (with hindsight there might have been better options): I physically removed (!) the HDD, installed a 2nd copy of Ubuntu 20.04 on a new separate partition on the SSD, again in "expert mode". During installation I was warned that there was no EFI partition and the system would be unusable, so I manually created an EFI partition for /boot/efi on the SSD. I then replaced the relevant lines in /etc/fstab of the first attempt with the correct ones from the 2nd. After a succesful reboot into the fixed system I added the old HDD again and manually ran "apt-get --reinstall install grub-common os-prober grub-efi-amd64" to rescan the HDD for extra grub entries.

To prevent others from running into this problem it would make sense to add another warning to the "expert" install process: there already is a check for the existence of at least one EFI partition in the system - this check should also warn if the EFI partition is not on the device that the user has explicitly selected for the boot loader.