how to move photos to external device

This documents describes how to prepare your external drive and then to move the complete library to the external drive: Move your Photos library to save space on your Mac - Apple Support

It is essential to prepare the external drive for compatibility with photos, or Photos may not be able to open the library there. Particularly the file system format needs to be MacOS Extended (Journaled) or APFS, the drive needs wired connection (not a connection over the local network), and the the drive must not be a used for Time Machine backups.

Added:

Have you been using iCloud Photos with the Optimize Storage option enabled?

If your iCloud Photos Library has been using the "Optimize Storage" feature it will be incomplete, and there is no point in moving the incomplete library to an external drive.

In that case create a new, empty library on the external drive and let the library download from iCloud to the new, empty library.

• Quit Photos, if it is running
• Lauch Photos again while holding down the options key ⌥ and keep holding down the key, until you are seeing the dialog to select a library. Click the button to create a new library and select a folder in the external drive.
• When the new library opens, go to the Photos > Preferences (Or Settings) and in the General Tab click "Use as System Photos Library".
• Enable iCloud Photos in the iCloud tab, also "Download originals".
• Wait and let the download run.

You will now have your iCloud Photos Library on the external drive. Once the download is complete, you can disable iCloud Photos for this library, then start to delete photos from iCloud, for example by syncing again with the library on your internal drive and deleting the photos from there.

Where are these photos located exactly?

If using the Photos App, you have 2 choices.

1. Export the photos from the Photos App to a folder on the external drive. Click Edit ➜ Select All, then Click on File ➜ Export. This will not preserve any albums or folders you may have created in the Photos App.
2. Move the entire Photos Libary.photoslibrary file from the internal drive under the Pictures Folder to the external drive, and then hold the option key when opening the Photos App to select the photo library from the external device to be opened. (the external drive must be formatted as HFS+ or APFS). This will preserve all albums, photos and edits you may have made.

If storing the photos in a folder in Finder, you can simply drag and drop the folder onto the external drive. To then view the Photos, just open the external drive in Finder, and navigate to the folder you copied there.

In iCloud how?

If you are using the iCloud Photos Option, then as I said, transfer the complete Photo Library as explained above to the external drive, then turn off the iCloud Drive Photos option in the Photos App under the Photos Menu ➜ Settings ➜ iCloud and then delete the photos through the iCloud.com website.

If you just stored the photos directly in iCloud Drive, then drag them from the iCloud Drive folder in Finder onto the external drive.