I have a Dell Inspiration 15 5555 with Elementary OS Hera which has a base 1Tb HDD, but I wanted upgrade it and recently install a 120Gb SSD using a caddy replacing my DVD/optical drive. Now I want to move my only the OS from the HDD to the SSD but keeping my Documents on the HDD. How can I do it? All the information I can find on the web migrates all the data and my documents are much bigger than 120Gb plus those doesn't need so much read/write speed as the one the SSD offers. Already format the SSD using ext4 but I can't find how to move only the OS. If it's need I still have the OS bootable with Hera. Been using Elementary OS for 2 years so theoretically I should understand it but to be honest most of the time I have no idea what is going on so please help me.
So do you mean that you want upgrading to an SSD, but you insist on keeping your current OS? I think you can try to clone your HDD to SSD. maybe this article is quite helpful