[Solved] Mint boot broken after installing Linux Light to external drive

[BanAnna2019]

Hello everybody.
This is my first post for a long time. I used to have another account many years ago, but do not seem to get into it anymore. There probably is a solution to my problem here somewhere, but I do not want to try something and then mess up things more then they already are.

While I have been using Linux for many years, I have a very limited knowledge of the command line. I practically have to look up everything on the Internet.
Laptop is Lenovo T460s. I am running Linux Mint Cinnamon 21.3.
My laptop is dual booting with Windows 10 and Linux Mint on one hard-drive. I have updates, Internet access, disabled on Windows 10.
I only plan to use Windows with a few programs I am not able to use on Linux.
2 TB hard-drive. Windows is 250 GB and was installed first. Linux Mint 300 GB, installed second. Rest is storage.
Problem: I tried to create a persistent Linux Light installation on an external drive. I created a live USB of Linux Light, rebooted into this and installed to an external SSD.
After I removed the external hard-drive with the Linux Light installation, I was unable to boot into my main OS, Linux Mint and got the following message:

GNU GRUB version 2.06
Minimal Bash-like line editing is supported. For the first word, TAB lists possible command completion. Anywhere else TAB lists possible device or file completions.

grub>
grub>

I can get into Mint again when I attach the external Linux Light. I then get a normal boot screen showing Linux Light as my main OS. This also gives me the option to boot into Mint. Once there I realised that I was not able to see my storage partition or my Windows 10 anymore.
Windows 10 does not seem to be affected. I can boot into it through BIOS, even when the external Linux Light is not attached.
I am currently writing this from my Linux Mint installation.


I ran "sudo parted -l; lsblk -f" both with the external Linux Light attached and without it.
Unfortunately I do not seem to be able to upload the results as I have no option to attach anything from my computer here.
I hope somebody will be able to help me regardless.
I have not been on any forum for years, so please excuse any mistakes I have been making in this post.
I actually am quiet desperate. I urgently need this laptop.

Thanks Heaps In Advance.

[BanAnna2019]

I was wrong about my Linux Mint not recognising the other partitions on the drive. My Mint still does recognise the other partitions.

[diabolicbg]

Welcome BanAnna2019!
This is just an idea, I don't know for sure what went wrong.
- Make an installation flash with LM.
- Boot the computerwith this flash drive WITHOUT TURNING ON THE EXTERNAL DISK!
- Select Boot-repair from the live session menu and start it.
- Restart the computer.
After this operation, the LM should boot. If it does, open Terminal and enter the command:
sudo update-grub
This will create an OS selection menu on the computer itself.
Restart the computer.

[BanAnna2019]

Many, many thanks diabolicbg.
I only got round doing something about this today and it worked!
Incredibly happy about this. I will never again test anything new on this laptop. I have an old laptop that I was planning to use for things like this and then didn't. Lol.

[diabolicbg]

The pleasure is mine. Glad you made it!