After installing recent updates in my Ubuntu 18.04 via Software Updater, Grub2 boot-loader menu is not showing on startup. The dual-boot (or triple-boot) operating system is no longer available when you press Shift (or Esc) to show the menu entry.

It seems to be a grub2 package issue in the recent update, though I’m not sure. And sudo update-grub can’t detect operating systems outside the current Ubuntu release.

If you happened to have the similar issue, try following steps to fix the problem.

1.) Open terminal and run following commands one by one to install grub-customizer:

sudo add-apt-repository ppa:danielrichter2007/grub-customizer

sudo apt update

sudo apt install grub-customizer

For Ubuntu 19.04 and higher, grub-customizer can be easily installed from Ubuntu Software.

2.) Launch Grub Customizer. When it opens, do:

1. navigate to General Settings tab.
2. check the first two check-boxes under visibility to show menu, check for other OSes.
3. click on the Save button.

3.) After saving the changes, navigate to ‘List configuration‘ tab, and click on Refresh button.

The Grub2 boot-loader should finally list all available operating systems menu entries in the next startup.