Boot into systemd-boot alongside grub

[juba0x00]

• I have installed EndeavourOS alongside Manjaro, it uses systemd-boot, not grub.
  I can’t boot into EndeavourOS, it does not appear in the grub menu.

[Aragorn]

Try changing the line…

GRUB_DISABLE_OS_PROBER=true

… in your /etc/default/grub into…

GRUB_DISABLE_OS_PROBER=false

… and running…

sudo update-grub

… from within Manjaro.

[juba0x00]

I found it false

# Uncomment this option to enable os-prober execution in the grub-mkconfig command
GRUB_DISABLE_OS_PROBER=false

this is the whole configuration

GRUB_DEFAULT=saved
GRUB_TIMEOUT=5
GRUB_TIMEOUT_STYLE=hidden
GRUB_DISTRIBUTOR="Manjaro"
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash udev.log_priority=3"
GRUB_CMDLINE_LINUX=""

# If you want to enable the save default function, uncomment the following
# line, and set GRUB_DEFAULT to saved.
GRUB_SAVEDEFAULT=true

# Uncomment to disable submenus in boot menu
#GRUB_DISABLE_SUBMENU=y

# Preload both GPT and MBR modules so that they are not missed
GRUB_PRELOAD_MODULES="part_gpt part_msdos"

# Uncomment to enable booting from LUKS encrypted devices
#GRUB_ENABLE_CRYPTODISK=y

# Uncomment to use basic console
GRUB_TERMINAL_INPUT=console

# Uncomment to disable graphical terminal
#GRUB_TERMINAL_OUTPUT=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command 'videoinfo'
GRUB_GFXMODE=auto

# Uncomment to allow the kernel use the same resolution used by grub
GRUB_GFXPAYLOAD_LINUX=keep

# Uncomment if you want GRUB to pass to the Linux kernel the old parameter
# format "root=/dev/xxx" instead of "root=/dev/disk/by-uuid/xxx"
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
GRUB_DISABLE_RECOVERY=true

# Uncomment this option to enable os-prober execution in the grub-mkconfig command
GRUB_DISABLE_OS_PROBER=false

# Uncomment and set to the desired menu colors.  Used by normal and wallpaper
# modes only.  Entries specified as foreground/background.
GRUB_COLOR_NORMAL="light-gray/black"
GRUB_COLOR_HIGHLIGHT="green/black"

# Uncomment one of them for the gfx desired, a image background or a gfxtheme
#GRUB_BACKGROUND="/usr/share/grub/background.png"
GRUB_THEME="/usr/share/grub/themes/manjaro/theme.txt"

# Uncomment to get a beep at GRUB start
#GRUB_INIT_TUNE="480 440 1"

# Uncomment to ensure that the root filesystem is mounted read-only so that
# systemd-fsck can run the check automatically. We use 'fsck' by default, which
# needs 'rw' as boot parameter, to avoid delay in boot-time. 'fsck' needs to be
# removed from 'mkinitcpio.conf' to make 'systemd-fsck' work.
# See also Arch-Wiki: https://wiki.archlinux.org/index.php/Fsck#Boot_time_checking
#GRUB_ROOT_FS_RO=true

[dragan]

Also make sure your EOS partition(s) are mounted - you need to do this in some cases e.g. when your other OS is encrypted. If you can see all its files from the terminal OS prober will pick it up (if the above flag is false). If you have multiple e.g. boot, root, home partitions, mount them all (I guess boot and root count but not sure).