I have a MacBook Pro running Yosemite and my Photos Library is taking 235G of space so I want to back it up into an external hard drive and delete it from MacBook Pro. I am kinda new to Mac OS systems and never really understand how the Photos application works.
I have my iPhone backups on this same MacBook Pro and have also done picture/video imports via the Photos Library application.
The Photos application shows my pictures / videos from 2007 up to 2016.
But I am confused that when I get into Show Package Content of the Photos Library and then the Masters folder, I only see year folders of 2015 and 2016.
So, my question is: From where is the Photos Library pulling together all of my media files? As you know, the Photos Library software does not allow us to right click a picture or video and reveal in finder, which is very annoying.
The bottom line question is: If I drag and drop the Photos Library package to an external drive, will I have successfully backed up my media and safe to delete them from my macbook pro? 
Best Answer
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 & 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 & you can delete any & all external files, should you wish.