linux – Changing ESP mount point from /boot to /efi

The problem isn’t that the ESP mountpoint moved, but that the OS kernel moved with respect to the bootloader (which is still in the ESP).

Your bootloader needs to know where the vmlinuz kernel image is (as well as the adjacent initrd or initramfs), and since it runs before Linux, it cannot rely on the Linux paths for that. So even though the file still gets installed at /boot/vmlinuz-linux from the Arch point of view, that now corresponds to a different partition physically (and potentially a different filesystem type), so the grub.cfg needs to be updated accordingly.

In other words, you need to repeat the GRUB installation: chroot from Debian (or from an Arch USB), then re-run grub-install (so that it would bring in the GRUB modules necessary to access your / filesystem – especially if it’s on LVM/LUKS/Btrfs), finally re-do grub-mkconfig (so that it would generate a grub.cfg that knows where the kernel is).

Make sure to specify --efi-directory=/efi when installing GRUB as it won’t auto-detect that.