How can I safely backup and remove Photos from MacBook (El Capitan)?
I have a simple solution for that:
Make sure to hold option key while clicking on Photos before lauching it.
Choose the folder where your different libraries are stored (if you want to create different libraries check this out
 Then, make sure to be in the Photos stack and select Command + A to select all the pictures. 
Head to File > Export (or click on Command + Option + E).
Don't forget to specify your external files format and videos definition.

Then you can choose to export your original or modified pics on your external hard drive.

Hope it helps ;)