Photos on external hard drive

First make sure the drive is formatted correctly. It must be as should be as shown in this screenshot:

Then just drag the library from your Pictures folder to the external HD and let it be copied. When the copying is completed double click on the library on the EHD to open and verify that all is well before deleting the one in your Pictures folder.

Close Photo and move the photo library to another disk.

Then restart Photo holding the Option key and chose the location of the library.

Have a look here: Designate a System Photo Library in Photos - Apple Support or this as a quick tutorial: How to Move Your Apple Photos Library to Another Location