How reinstall Ubuntu 18.04 on single boot system and keep personal data

[Impavidus]

The boot info summary tells us that you've got a fairly standard system. Unfortunately, the information is already outdated as you attempted to install Ubuntu on sdb.

There's something weird about your screenshots. You've got mbr partitioned hard drives and you old system in installed in legacy mode, but your new install on sdb created an EFI partition, suggesting you installed in UEFI mode. Best not to mix modes.

You can use a dirty reinstall. You can use manual partitioning ("Something else"), select sda1 to use as root partition and NOT format it. That will reinstall Ubuntu, leaving your documents untouched. It's more convenient though to have your documents and the OS in separate partitions.

But I think right now it's best to use a clean install, as TheFu suggests. Copy your documents to a backup drive, disconnect the backup drive, install, restore your backups. If you wish, you can take the opportinuty to repartition your drive.

[pbear42]

You can use a dirty reinstall.

Was going to suggest that option. One way to make it cleaner: Delete all the directories except /home.
Now, don't have to rely on stuff being overwritten (or harmless if not).

[itsmarco]

I did a "dirty" reinstall. Moved my home direction back and added the files to new created one. Most of the data was available except some hidden folders and applications (like the thunderbird profile, etc). I wasn't so hard to get all programs and settings back and it took me less time in finding the error from the problem before.

Thank you form your help a lot! I guess for the future I will separate the /home directory and I learned a lot about the linux system in general.