Comment 6 for bug 1886148

Furthermore, I tried to make a persistent live drive with mkusb (mkusb-dus) using the default settings, which has been working for years (until a few days ago also in Lubuntu Groovy).

It failed completely in BIOS mode at the grub rescue prompt :-(

It reached a live-only working desktop in UEFI mode :-/

It does not work at all using mkusb with 'usb-pack-efi', another strong indicator that the boot structure is not what it has been for a long time.

So it seems that either the boot structure has changed a lot by intention, or there is some severe bug. I think we an explanation from a developer at Canonical. I would guess that there are big changed in the package casper. (I don't think that the Lubuntu team is responsible for this bug.)