Hi, and thanks for your reply.
The collection was a Photos library. Not an old one, as far as I know. I am running Catalina so I assume it's all up to date. There were other items in the Pictures folder, too, including an old iPhoto library, but my main concern is the current Photos library (which contains everything that was in the old iPhoto library plus newer content).
To move the user, I transferred the whole home directory to the external drive using the technique set out here:
https://www.idownloadblog.com/2017/01/25/how-to-move-mac-user-folder-to-another-drive/ The user was moved entirely, kept intact. I then used "Advanced Options" in Users and Groups to point to the external drive for the home directory. Exactly as shown in the link.
The external drive is formatted as Mac OS Extended (case-sensitive, journaled). I don't know the answer to your question about file ownership. When I use Get Info it doesn't show me the check box for "Ignore ownership on this volume". Under Sharing & Permissions it says "You can only read" and has Read & Write permissions for System and Wheel. Not sure if that is helpful information to you...
Thanks again for replying!