Fix missing dual boot menu with Windows 10 and Debian on Acer laptops

Lately I went through the struggle of installing Debian alongside a pre-existing Windows 10 on a UEFI-powered machine (Acer Aspire E15). The Debian installation went fine, but I wasn't able to boot into Debian: no OS selection menu came up upon boot. What to do?

Step 0: make sure you are installing Debian the right way

Different tutorials on the Net suggest different routes. In general, you need to tweak two things before installing Debian: disable the fast startup on Windows 10 and disable secure boot in UEFI.

Disable fast startup on Windows 10

Fast startup is sort of partial hibernation of files and data. Unfortunately this creates a problem when you want to copy stuff from Debian to Windows. So let's disable it:

1. open "Control Panel";
2. select "Power Options";
3. click "Choose what the power buttons do";
4. click "Change settings that are currently unavailable";
5. scroll down to "Shutdown settings" and uncheck "Turn on fast startup";
6. click "Save changes";
7. reboot.

Disable secure boot in UEFI

Secure boot is a way to stop malicious software to replace your boot loader. Unfortunately this makes Linux difficult (or impossible) to install. How to disable it:

1. Reboot the system;
2. press F2 to access the UEFI settings;
3. navigate to "Boot" menu;
4. disable "Secure Boot" as shown in picture 1. below;
5. save, exit and restart.

1. Acer's UEFI boot menu: where to disable the Secure Boot option.

Step 1: Select the proper UEFI file to load

Now you can install Debian as usual. Once finished, the new OS will be correctly installed but not yet accessible at startup. So head back to the UEFI settings (i.e. reboot and press F2) and in the "Security" tab select select an UEFI file as trusted for executing, as shown in picture 2. below. Here you are telling UEFI that there's another OS to take into account.

The interface will display a directory: browse it down until you find a file named BOOTx64.EFI, then select it. You will be able to choose a name too: that's what will be displayed in the boot order list later on.

The menu seen in picture 2. might be greyed out. If so, temporarily re-enable the secure boot as you did in the previous point: it should unlock the menu. Remember to disable secure boot once finished.

Save, exit and restart.

2. Acer's UEFI security menu: select an UEFI file as trusted for executing.

Step 2: define a proper boot order

Take a look at picture 1. above: you will see how my boot order still gives priority to Windows Boot Manager (#9) over Debian (#10). Unfortunately the Windows Boot Manager is unable to see other OSes such as Debian and the like. If you leave it as it is you will boot straight into Windows 10 with no OS selection. You need to switch to grub, the smarter Linux boot loader which is able to handle a wider selection of operating systems.

To change that, enter the UEFI settings again (reboot + F2) and flip the boot order of Windows Boot Manager with EFI File Boot 0: Debian. Save and exit. This way, the next boot you will be prompted with the OS selection managed by grub.

Sources

Acer Answers - Windows 10: Enable or disable fast startup (link)
CNet - What is Windows 10's fast startup? (link)
Ask Ubuntu - Why disable Fast Boot on Windows 8 when having dual booting? (link)