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I am having some trouble partitioning. I'll try to explain everything from the beginning. I have two laptops, 1 and 2. On laptop 1 I have only Ubuntu 18.04 installed, and I made a backup of my system by following the instructions here. On laptop 2 I installed Gentoo on dev/sda, following a tutorial on Youtube. The partitions dev/sdaX for X=1,2,3,4 are boot,grub,swap,root, respectively. The Gentoo installation completed successfully - it would boot up to a CLI fine and I could move around and edit files.

I tried to do the following: Install Ubuntu alongside Gentoo on laptop 2, and restore the copy of laptop 1 onto this partition. First, I had to resize some partitions. In the Gentoo install, I made sda4 take up the remainder of the drive. On the Ubuntu live disk I ran Gparted and resized sda4 to take up around 20 GiB, and made the remainder of the drive (partition sda5) available for Ubuntu.

During the Ubuntu installation I chose "install Ubuntu alongside Gentoo", and divided it to save around 20 GiB and leave the rest for Ubuntu. I misunderstood this part - the net consequence was that sda5 was resized to 20 GiB, and the remainder of the drive (sda6) contained Ubuntu. The rest of the installation was successful, and I restored my backup from laptop 1. It seems like it completed successfully, because I can mount sda6 and all the right files are there. Importantly: at this step I had booted from grub into the new Ubuntu install, and Gentoo was still available from the menu.

I tried to reboot, and got a GRUB menu that only had Ubuntu in it. Confusing, but ok. I tried to boot into Ubuntu and got the following errors:

error: no such device XXXXXX
error: no such partition
error: you need to install the kernel first

I ran the boot-repair script and it said it failed; I got the following pastebin: http://paste.ubuntu.com/p/nJ3r2dZ5sy/

Restarting, my boot menu now has Ubuntu and Gentoo available. I have tried to boot into Gentoo and got in just fine. I tried to boot into Ubuntu and the power up dots are lit, but it seems to hang there. If I tap the power button then it shuts down in a safe-looking way, but it doesn't get to the login screen or anything.

If it's possible to tell, did I do anything wrong? If so, what, and how do I fix it/avoid it in future? I still have my backup so reinstalling ubuntu on sda6 or whatever and restoring again is an option. But I'd like to understand it a bit better.

  • Very happy to provide diagnostics if people tell me what to run - I'm not sure what is helpful to see. – preferred_anon Mar 2 '19 at 17:07
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Booting into Ubuntu would not work, but I could boot into recovery mode. From there, running fsck gave me an error like Could not find partition XXXX, which fits with the message I was getting before.

It turns out that every partition has a so-called UUID, which is stored in the /etc/fstab file. When I changed my partitions, the UUID for my root partition did not match the one in /etc/fstab. To find out the correct one, I did

blkid /dev/sda6

in the root command shell in recovery mode, which gave me a different id, call it YYYY. Now, the problem was solved by opening the /etc/fstab file (which was initially read-only, even as root, but the second answer to this question resolved that problem) and changing the UUID there to the correct value. I copied the line that was above it apart from that.

And now I can boot into Ubuntu!

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