Using external drives

If you are asking which external drive holds the library you currently have open, look in Photos > Settings... > General > Library Location for the full path to the library. (See brbo's comments if the external drives have the same name.)

Re: "Using external drives: In photos how do I know witch external drive I am using as I have two on the desk top"

The external drives likely appear differently, or are named differently, on their desktop icons.

You could double-click each of the external drives icons on the desktop, to see what files are in the folders on each.

If you are using one for photos ... you will soon know which one. (You could "re-name" the drives if you wish.)

Thank you kindly for replying, and for letting us know helped! All the best :-)