Photos application and backup on iMac with macOS Sierra

If you already know where your Photos library exists you could drag that to another drive and copy the contents to that new location/drive. If you don't know where your Photos library is located you could launch Photos, then select Photos/Preferences from the Menubar. From the new window Toolbar select General and Show in Finder. This will locate your currently used Photos library. Copy that to a new location and create a backup of the original photos library - including modified and original photos.

Yes you may have multiple Photos libraries. You will need to "tell" Photos which one you wish to use by holding down the Option key when you start Photos. A screen will pop up asking you to select the proper library you wish to access. Furthermore, a selection needs to be made to "tell" Photos which is the System library. That is, which Photos library will pop up when you access the media icon shown in various other applications. Hope this helps.

PS. You will know that you are copying a file because a green + will appear when you move the file. if that green + symbol doesn't appear you are merely relocating the file, not copying it.

Maciej194 I would like to help with this question, but am not sure I understand it well enough. If you upgrade to a newer version of OS, then erase the drive with it on, you will have undone all the work and time you spent putting the new OS on the drive. Similar to decorating your driveway with chalk, then turning the hose on it. A waste of time, right?

Perhaps if you repost your question using a different header/subject line you will get more attention and a better answer.