OS Prober no longer ran by default as of today's update of grub

[Stephane]

see this topic

Warning: os-prober will not be executed to detect other bootable partitions. Systems on them will not be added to the GRUB boot configuration. Check GRUB_DISABLE_OS_PROBER documentation entry

edit /etc/default/grub add or uncomment GRUB_DISABLE_OS_PROBER=false save that file then run sudo update-grub or sudo grub-mkconfig -o /boot/grub/grub.cfg