If you have, i.g., another linux distro, such as KDE-Neon, and you want add Kubuntu as secondary SO, without changing your EFI, you do this way:
- in Kubuntu (usb iso) menu -> try Kubuntu
- in a terminal (once logged in):
ubiquity -b - this will open the graphical installation tool letting you install Kubuntu without modifying existing EFI
- don’t worry for the warnings (“the system could not start”)
- after rebooting, in KDE-Neon do
update-grub
If you get, with update-grub, an error such this:
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
You can solve adding or uncommenting this line in /etc/default/grub
GRUB_DISABLE_OS_PROBER=false