how to dual boot fedora 27 and fedora 28 [closed]

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I have done this for quite a while, and it works without modifying /etc/grub.d/40_custom. I don't have UEFI so this is pure MBR booting. You may need to run fsck on the root file system from the other system before running grub2-mkconfig.

I could also select to boot from the other disk in the BIOS boot setup, and that workes perfectly as well.

os-prober is used by grub2-mkconfig to locate the other installed operating systems. Following shows that it found one.

# os-prober 
/dev/sdb5:Fedora 28 (Twenty Eight):Fedora:linux

grub2-mkconfig will then read the information from sdb5 and sdb2, which in this case contains the boot partition corresponding to the root partition sdb5

# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found theme: /boot/grub2/themes/starfield/theme.txt
Found linux image: /boot/vmlinuz-4.16.5-300.fc28.x86_64
Found initrd image: /boot/initramfs-4.16.5-300.fc28.x86_64.img
Found linux image: /boot/vmlinuz-4.16.4-300.fc28.x86_64
Found initrd image: /boot/initramfs-4.16.4-300.fc28.x86_64.img
Found linux image: /boot/vmlinuz-4.16.4-200.fc27.x86_64
Found initrd image: /boot/initramfs-4.16.4-200.fc27.x86_64.img
Found linux image: /boot/vmlinuz-4.16.3-200.fc27.x86_64
Found initrd image: /boot/initramfs-4.16.3-200.fc27.x86_64.img
Found linux image: /boot/vmlinuz-4.15.17-300.fc27.x86_64
Found initrd image: /boot/initramfs-4.15.17-300.fc27.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-dc5c0cbdda03c984024a73e54bad769b
Found initrd image: /boot/initramfs-0-rescue-dc5c0cbdda03c984024a73e54bad769b.img
Found memtest image: /boot/elf-memtest86+-5.01
Found Fedora 28 (Twenty Eight) on /dev/sdb5
done

Notice this line Found Fedora 28 (Twenty Eight) on /dev/sdb5 which tells me it created menu entries for loading linux from sdb5.

Keep 40_custom as it was at install time:

# cat 40_custom
#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.