Find referenced file

The support for referenced files in Photos is limited.

You can import photos as referenced, but there is no way to relocate referenced files to a new location, and there is no panel to reconnect missing files like Aperture had.

The only supported action is "Consolidate" to copy the originals into the library.

If a referenced file cannot be found, you have to search for it on your own and point Photos to its location.

The question remains, why Photos cannot access your referenced files.

Is your external drive locally mounted and formatted MacOS Extended (Journaled)? On a NAS you may have problems with the referenced files on Sierra.

In Photos it is much safer to consolidate all referenced files into the library, and then use File > Export to create a folder with copies of the photos, if you need to access them outside Photos.

Unfortunately Photos does not handle referenced files well adn they are not recommended - there is no ability to relocate referenced files with Photos and many other reasons not to use them - Disadvantages of a Referenced Library in Photos

You need to rethink your use of referenced photos and strongly consider consolidating them into your library - it is the recommended way to use Photos

LN

If there is a pop-up window asking for the folder, you can point it to where the referenced images are. This comes always up when you changed the file's place.

That is exactly what I meant by "limited" support for referenced files. The pop-up window does not tell you anything about the missing file, and not where the file used to be, just the filename. Aperture has an elaborate reconnect window, where you can compare the file attributes of the missing file with any file you found. And you can automatically reconnect all files found in the similar location.