photos

Is the libraryon the external drive your system photos library? If yes, keep the external drive connected at all times, when you are signed into your user account. The external drive needs to be connected before you log into your user account, or Photos may lose the connection to your system photos library and will need to restore it, before you can use it again. and do not use the library from different user accounts, unless the "Ignore ownership on this volume" flag is set for the volume with your Photos Library. See: Move your Photos library to save space on your Mac - Apple Support

What format is the EHD? It needs to be formatted Mac OS Extended (Journaled) or apfs. How are you connected to it? It needs to be USB or Thunderbolt, and not a network link.