The company laptop initially had Archlinux installed with an encrypted root inside LVM on a LUKS partition with lots of free space left over for the installation of Ubuntu 15.10. Installing Ubuntu as the second OS on an EFI boot machine ran into two problems:
1. If you choose to install Ubuntu 15.10 into a new LUKS partition on a disk already containing other LUKS partitions, the Debian Installer will erase all other LUKS headers when it creates the new LUKS partition. In fact, this is a known issue and the developer of cryptsetup warns (section 1.2 WARNINGS) against installing Ubuntu with LUKS on a disk that already contains other LUKS partition:
UBUNTU INSTALLER: In particular the Ubuntu installer seems to be quite willing to kill LUKS containers in several different ways. Those responsible at Ubuntu seem not to care very much (it is very easy to recognize a LUKS container), so treat the process of installing Ubuntu as a severe hazard to any LUKS container you may have...the installer offers to create LUKS partitions in a way that several people mistook for an offer to activate their existing LUKS partition. The installer gives no or an inadequate warning and will destroy your old LUKS header, causing permanent data loss
2. Ubuntu can fail to install grub2 into the ESP (EFI System Partition) if another boot manager has already been installed there. I think this occurs because the path /boot/efi doesn't exist, which can happen if another linux distro (like Archlinux) is first installed and a boot manager like bootctl has created the path /boot/EFI on the ESP. The sole difference is EFI in all-caps or not, but this seemingly-minor issue causes problems for the Ubuntu installer.
I therefore strongly recommend that you install Ubuntu as your first OS before installing other Linux distros to create a EFI multiboot system. Archlinux plays well with other EFI boot managers installed to the ESP.
During the installation of Archlinux as the second OS on a EFI boot machine, the ESP will be mounted as /boot (actually /mnt/boot before chrooting into the new system). To install the bootctl boot manager, simply invoke
inside the chroot, which will then install bootctl into the ESP.
I created Archlinux as my default boot entry at /boot/loader/entries/arch.conf :
options cryptdevice=UUID=b332de40-afe8-47d5-9512-bf03da8d13cc:ARCH root=/dev/mapper/ARCH-rootvol quiet rw
The example above includes boot options for opening a LUKS partition and accessing '/' inside LVM.
Now you also need to create an entry for ubuntu so that bootctl will load grub2 menu for 15.10 Wily. Let's call it /boot/loader/entries/ubuntu.conf:
title Ubuntu 15.10
Finally, you need to add the Ubuntu 15.10 entry to /boot/loader/loader.conf:
Now when you get the to EFI boot menu, you will see entries for both Archlinux and Ubuntu.