A year or so ago I installed Ubuntu 20.04 LTS on an Acer laptop (SSD system disk) and also made a rescue/safe experimenting FAT32 partition on the internal HDD with the contents of ubuntu-20.04-desktop-amd64.iso as if it were a bootable USB-stick, by copying the contents of the stick to the partition. Now I can start an Ubuntu live session via the F12 alternative boot selection and I have a Grub2 entry in my normal Grub boot menu for starting a live session from the partition. Recently (after updates?) the Grub start got broken when secure boot is active. It still works via F12 and via Grub if secure boot is disabled.
When starting I get the following errors:
error: vmlinuz/casper has invalid signature
error: you need to load the kernel first
The entry in /etc/grub.d/40_custom is as follows:
menuentry "Ubuntu 20.04 LTS live session (skip install window)" {
insmod search_fs_uuid
insmod chain
search --fs-uuid --no-floppy --set=root 7ca7-318e
chainloader ($root)/EFI/BOOT/grubx64.efi
linux /casper/vmlinuz boot=casper quiet splash
initrd /casper/initrd
}
I also tried filling the partition with contents from the latest ubuntu-20.04.2.0-desktop-amd64.iso version, keeping 40_custom the same. The booting started up to the rotating wheel, but ended with an avalanche of messages. Starting via the F12 menu works OK.
How can the invalid signature be resolved - or - how can 40_custom be changed to start using the contents of the latest .iso version - assuming that that this solves the invalid signature issue?
