Importing Photos from Lion to Yosemite Photo Library to External Storage Device
From Apple Support:


  To migrate another photo library manually:
  
  
  Quit Photos. 
  Hold down the Option key and open Photos.
  Select the library you want to open. 
  If the library you want isn’t listed, click Other Library. Navigate to the library you want and click Open.
  
  
  You can also drag the library you’d like to open to the Photos icon in the Dock. Or you can click Create New to create an all-new, empty library. 
  
  Photos can use libraries from iPhoto 8.0 or later and any version of Aperture. If you’d like to migrate a library from iPhoto 7 or earlier, use Apple’s free iPhoto Library Upgrader tool to prepare your library first.