File upgrade and cleanup went well, I always choose default options.
There was just one little issue at startup:
"ubuntu disk drive for /media/sdb1 not ready yet or not present."
I could skip this with the "S" key and start the system alright. Hoever I thought it would be anoying to press this S key in the future at each startup. Thanks to this forum thread (where a very angry guy speaks) and this other site. I found the fix was to edit the file system table.
The following command starts the editor:
sudo gedit /etc/fstab
Then I commented out the line corresponding to this - inexisting - file system.
#/dev/sdb1 /media/sdb1 ntfs defaults 0 0
I saved the file and now I'm about to restart my computer.