Catalina weird behaviour with Photos - External disk not updating

It takes a very long time for the modification date and the size of the Photos Library to update, even with the library in the Pictures folder in your home folder. I have seen the same date and size there for months, in spite of working with my photos every day., and my library is on an APFS volume in my Pictures folder The internal folders inside the library package are showing the correct modification date, while the library is now showing a modification date for August 31. I am ignoring these date and the size since the Catalina upgrade.

But the long time to open the library seems to be a serous problem. How is the external volume connected to your Mac? Is it directly plunged in or accessed remotely? You need a locally mounted volume.

Has the external volume been used by Time Machine? Then you cannot use it for a Photos Library. As a test, try to set the "Ignore Ownership on this volume " flag for the external volume as described here: (Move your Photos library to save space on your Mac - Apple Support). If you cannot set this flag, the volume cannot be used.

The external volume should always remain connected to the Mac, as long as you are signed into your user account, even if you are not working with Photos. The background processes will try to access the library while you are not using Photos, and if you disconnect the volume and plug it only in, when you want to work with the library. it will slow Photos down.

anyway there is still "analysing scenes & composing layouts", still has to scan 61000 photos for faces and maybe some etc. Probably I will have to worry if it's not finished on one week or so.

You can use Photos while this process is running. All it does is analysing the photos to find the main objects in the photo and to identify your best photos. This is used to create the tiled layout of the "Days"view with only the best photos, without duplicates. On my MacBook Pro with 53000 photos in the main Photos Library this process has never finished between system updates, only for my smaller test libraries with only a few thousand photos. But all other functions of Photos have been working properly, even if the curating layout never finished since the Catalina upgrade last year.

If it hangs for weeks, without making progress beyond 90% or so, you may have videos or photos in your library that photos cannot process. I have not yet finished to convert all videos to a different format. Look for older AVI videos, for PDF files, for Audio files (any legacy media from this list: About legacy media in iMovie for macOS - Apple Support).