I have Ubuntu 18.04 in my working SSD. I wanted to migrate to a new Ubuntu 20.04. So I installed a new Ubuntu 20.04 and working in it. Now the old Ubuntu 18.04 was taking a lot of space in my working SSD. But I needed it in case if I needed anything from my old workspace. So I took a partition snapshot/image using dd and restored it in a separated partition in my external hard disk (which before I only used for data, never OS installation).
Then I booted from my eHDD expecting to get into my Ubuntu 18.04 but it didn't work because of some grub issues. Then I did "grub-install sdX" to my eHDD. Now when I boot from my eHDD the grub is showing the list of OSes in all my drives (2 internal and 1 external drive. don't know why). As it shows my eHDD Ubuntu 18.04 I have nothing to complain. If I select any OS it booted well, but if I choose my eHDD Ubuntu 18 it is booting from my original SSD Ubuntu 18.04. Trying to troubleshoot it I installed a fresh Ubuntu 20.04 in my eHDD and it is booting good.
So why is my Ubuntu 18.04 image not booting. May be I should do some setting since its an image restoration not a fresh install?