MacOS – how can I add photos from an external drive to the new Photos

If you simply want the entire Masters structure copied to a new drive, then

• first find your iPhoto or Photos Library file. Default location for both is ~/Pictures/

• Right click the Library file & select Show Package Contents - that will expose the 'file' as the folder it really is.

• The Masters folder should be just inside. You can copy that to anywhere.
  It is sorted into years, then months. Not all contained folders will actually be populated, if old photos were ever deleted, the file structure remains.

As pointed out in comments, if you need the copied structure to be readable natively on Windows, then you ought to format the target disk as ExFAT.
Windows cannot natively read Mac OS Extended disks & would need 3rd party support.

From Apple Support:

To migrate another photo library manually:

1. Quit Photos.
2. Hold down the Option key and open Photos.
3. Select the library you want to open.
4. 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.