How to Install Manjaro Xfce alongside existing Ubuntu 20.04 installation?

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First, decide what partitions you want to use:

  • /
  • /home {optional}
  • swap {optional}

I would prepare the partitions upfront with Ubuntu (incl. formatting and flags).

Then install Manjaro using just assigning the prepared partitions in Calamares.

  • If booting Ubuntu in UEFI mode, boot the Manjaro live ISO in UEFI mode as well and assign the existing ESP for the boot loader.

  • If booting Ubuntu in BIOS mode, boot the live ISO in BIOS mode as well and assign the MBR as location for the boot loader.

If UEFI is considered, boot via firmware into the new Manjaro install.

Change the /etc/default/grub file to achieve following settings:

GRUB_TIMEOUT=10
GRUB_TIMEOUT_STYLE=menu
GRUB_DISABLE_OS_PROBER=false

Then run

sudo update-grub

reboot and enjoy!