If you have ever tried installing Ubuntu Server on a Mac Mini it’s likely that after installation you encountered  a flashing folder icon with a question mark on the screen. Below are the steps to fix it.

1. Boot into your installation USB by holding ALT key when the Mac Mini is starting.
2. Press C when the GRUB menu appears to start a console.
3. Now you have to find the partition that contains a boot directory.
   
   Shell

   grub> ls (memdisk) (hd0) (hd0,msdos) (hd1) (hd2) (hd2,gpt3) (hd2,gpt2) (hd2,gpt1) grub> ls (hd2,gpt2)/boot/grub unicode.pf2 [...] grub.cfg

   1 2 3 4 5

   grub> ls (memdisk) (hd0) (hd0,msdos) (hd1) (hd2) (hd2,gpt3) (hd2,gpt2) (hd2,gpt1)   grub> ls (hd2,gpt2)/boot/grub unicode.pf2 [...] grub.cfg

4. Set this as your root for further commands.
   
   Shell

   grub> set root=(hd2,gpt2)

   1	grub> set root=(hd2,gpt2)

5. The only way to boot properly is to use the UUID of the drive. Get it like this:
   
   Shell

   grub> ls -l (hd2,gpt2)

   1	grub> ls -l (hd2,gpt2)

6. Note down the UUID. You’ll have to type it manually in the next step.
   
   Shell

   grub> linux /boot/vmlinuz(press tab here).efi.signed root=UUID=(the UUID from above)

   1	grub> linux /boot/vmlinuz(press tab here).efi.signed root=UUID=(the UUID from above)

7. Set initial RAM disk and start booting.
   
   Shell

   grub> initrd /boot/initrd(press tab here) grub> boot

   1 2	grub> initrd /boot/initrd(press tab here) grub> boot

8. Your system should start normally now so log in and run the following commands:
   
   Shell

   sudo apt-get install efibootmgr sudo efibootmgr

   1 2	sudo apt-get install efibootmgr sudo efibootmgr

9. This will list something like the output below:
   
   Shell

   BootCurrent: 0000 Timeout: 5 seconds BootOrder: 0080 Boot0000* ubuntu Boot0080* Mac OS X BootFFFF*

   1 2 3 4 5 6

   BootCurrent: 0000 Timeout: 5 seconds BootOrder: 0080 Boot0000* ubuntu Boot0080* Mac OS X BootFFFF*

10. Change the boot order to start Ubuntu first and reboot.
    
    Shell

    sudo efibootmgr -o 00, 80 reboot

    1 2	sudo efibootmgr -o 00, 80 reboot

11. Your Mac Mini should be able to start Ubuntu Server now.