Photos cannot load edits for this image

If it is an intermittent problem, it might be problem with the program memory. How much RAM does your Mac mini have installed? Some other app might be hogging the available memory. You can test this by avoiding to run any other apps in the background, while working with Photos.

Have you run Disk Utility to check the external drive? And tried to repair the drive?

The drive might be slowly failing or there might be a problem with the cable or the port. If you can, try to use a different port, a different cable, try the drive on a different Mac.

Is there still any seagate Software installed on the drive? You can test, if any installed software is interfering by trying to set the "Ignore Ownership on this Volume" flag, if it is not already set. If you cannot set this flag, the drive needs reformatting.

kj1234567 wrote: …Ideally an Apple person with inside knowledge of the software would get involved and provide some suggestions, not just for my benefit but also others.

This is a user to user forum-- "Apple people" don't read this stuff.

I had a more serious problem some years ago with an even larger library that became unusable. I could still see my photos, but could not edit any of them, nor could I export them.

This usually means that the database is corrupted or that original files have been lost, as I said earlier. You didn't answer, "Is "Copy Items" checked in Photos' Settings>General?"

Since then I am keeping separate unedited and edited versions of my photos somewhere else, just to be safe!

That's good. Files of edited versions don't include all the information in the database, though, so it's good to occasionally copy the Photos Library itself to a separate drive. Time Machine is good for making fast, incremental backups. I do all those things with separate drives.

Btw. I have just installed an external SSD and moved my Photos Libraries to that one.

SSDs are fast! They work best with APFS format.

If there is a limitation on the size of the libraries and/or issues with the speed of the Photos database lookups and retrievals someone should tell the users!

Of course, scans and searches slow down with an increase in Library size, but there's no maximum size. On the other hand, it may be worthwhile to divide a large Library up. I have a Favorites Library that is the one I show other people. And there's a Nikon Library and other Archive Libraries for more complete sets of pictures, including "nearly the same" type pictures and fuzzy ones that have information I want to keep. I don't throw away picture files-- but I don't want to search through the less used ones to find a good picture. The Nikon Library is also active-- that's where I edit pictures from those cameras before moving the "favorites" to the Favorites Library.

After léonie suggested that limited RAM can be a problem, I'm closing most RAM hogging apps while using Photos.

Because the first Mr. forget off "live" in picture ! The seconds you are using a traditional camera or through apps, adjust the milk in the traditional camera the photo will not reduce the resolution and change its size, I think you should take a traditional camera and adjust the milk inside the apps, then you remember to link ICLOUD which photo you like the most and verify the face, remember to check again after completion.

Thank you so much !

I haven't had this problem-- it's not normal behavior. I routinely upload pictures from an SD card to Finder folders for archiving, and I import pictures from the Finder folders to Photos. I don't normally import RAW to Photos.

How is your external drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

If this drive is in a an incompatible format, stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

It sounds like the Photos database may be losing contact with the original picture files, or the database by be corrupting. First:

• Are you using iCloud Photos? Do you have Optimize Storage turned on for Photos?
• How much free storage do you have on your Mac? You should have at least 10% of your internal drive remaining free. Photos and the OS use this space for manipulating files.
• Is "Copy Items" checked in Photos' Settings>General?

To narrow things down, you can try these things, some of which you've already tried. But since the problem is intermittent and unpredictable, testing may be hard, and it may take awhile to see if each step works or not:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps long term by clearing caches and stuff. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

You can't stay in Safe Mode forever, but see if you can, try to stay there long enough to see if there's a change.

• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.

These two steps can be important:

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to temporarily move your Library out of your own user's Pictuers folder up to the general "Users" folder. 

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…