How to back up Photos Library to an external hard drive?
You can move the entire Library if you first Consolidate it from Photos' File menu.  

This will pull all externally referenced files into the Library, after which you can move it in its entirety to a new drive. Next Launch of Photos it will ask you where it went &amp; you tell it.

If you then keep the 'Copy items to Photos Library' checked, then in future it will always ensure they are kept within the Library &amp; you can delete any &amp; all external files, should you wish.