I use Ubuntu on my own PC. I usually install 20.04 and use it, and when I checked the website to do ROS, the one using 18.04 was the mainstream, so with M.2 SSD (Nvme) with 20.04 installed Installed Ubuntu 18.04 on another SATA SSD. At this time, I wanted to install 18.04 on another SSD with the M.2 SSD that had 20.04 installed removed, but I gave up because the M.2 SSD cannot be easily inserted and removed. So, with the M.2 SSD with 20.04 installed, I plugged in the SATA SSD formatted for 18.04 and started the installation. At this time, I chose the option to install using Live USB and format and install the SATA SSD at the time of installation. Other than that, I also turned on download of updates and automatic driver installation, and I think that everything else remained the default. After that, when I started the computer, a screen for selecting whether to start Ubuntu 18.04 or Ubuntu 20.04 appeared, and the selected OS started normally regardless of which one was selected. And I found that ROS boots normally in 20.04 as well as 18.04, so Ubuntu 18.04 is no longer needed, so I pulled out the SATA SSD and tried to boot Ubuntu 20.04, but "GNU GRUB version 2.02 A screen that only says "Minimal BASH-like ..." (partially omitted) appears, and ** Ubuntu 20.04 could not be started. ** I tried to boot Ubuntu 20.04 from this screen, but it didn't work.
After investigating, it seems that the most commonly used workaround for this phenomenon is to use software called Boot Repair to fix the broken grub.
So, install Boot Repair like this website (https://itlogs.net/ubuntu-grub-boot-repair/), and with the recommended settings, ** once with both Ubuntu 20.04 and 18.04 SSDs plugged in ** Boot Repair Was executed.
After that, after removing the SATA SSD of Ubuntu 18.04, when I booted it again, the screen to select Ubuntu 20.04 and Ubuntu 18.04 was displayed even though there should be no SSD of Ubuntu 18.04.
After that, I selected Ubuntu 20.04, started it, and ran Boot Repair again, and even after rebooting, Ubuntu 20.04 started automatically.
There is a possibility that dual boot is not related in the first place, but the behavior changes depending on whether you remove or insert the SSD of Ubuntu 18.04, so there is some change in the grub setting at the timing of dual boot (probably which OS to boot) I think that the discrepancy occurred because the setting for selecting or something was changed) and the change did not return even if the SSD of Ubuntu 18.04 was pulled out.
I felt like I could understand why dual boot is dangerous. I thought they wouldn't interfere with each other because they weren't partitioned, but that doesn't seem to be the case. Please be careful about dual booting.