Photos launches but won't show images.

I tried something easier. I mentioned that Photos has thousands of files, but many are invisible or in ~/Library. I went to the user Library of user one with a functioning Photos. I copied every file related to Photos to the equivalent folder of the user two Library. I logged off completely. I logged on, and Photos worked, though I had to reset preferences. I hope that what caused the problem is a one-time quirk.

I thank everyone who replied. Not all the info helped with this problem, but it was educational.

Each user has separate libraries that aren't shared. The user with the Photos problems typically accesses Libraries located on disk images. The disk images are on an internal hard drive that is not the boot drive. (I have a Mac Pro 2012 with four internal drives.) The disk images are correctly formatted, have correct permissions, and "Ignore ownership on this volume."

I have an idea for fixing Photos for my second user. If I delete user one (after a clone backup) and choose "Don't change the home folder," I assume that a new installation of the OS will apply to the second (and only active) user. All Photos files and Photos will be replaced, and Photos should work. I can then resurrect user one. Will this work?

I use disk images for a number of reasons. I can group related files, and sensitive ones are secure if I use an encrypted disk image. It is easy to copy a disk image to other drives. You can send a disk image over the internet as is. If the files were in a folder instead of a disk image, you would have to compress (zip) the files to send them. You will note that almost all downloads from Apple and other vendors are disk images. The Finder treats disk images like volumes, which sometimes is better than treating the contents as if they were in a folder. Disk images appear in the sidebar for easy access (if you adjust Finder preferences).

User two has the same problems with Libraries not in disk images. I run High Sierra and Photos 3. I don't use optimizing or antiviral software. I know the problem isn't Photos itself, because it works for user one. I suspect failure or deletion of one or more of the thousands of Photos-related files in the user two local /Library. Since I can't fix that missing or damaged files, I want to know how to reinstall of High Sierra and replace the /Library contents of user two and not just user one.

I considered switching to a new user and created one. Then, I stopped. The new user would get a new user Library. I would lose all my preferences and all data (such as Notes) that Apple stupidly stores in Library containers, which violates a longstanding rule that the System folder should not contain files with data changed frequently by the user. I could hunt down those files, but it would be a chore. I can start the reinstall OS process just before I do something else (such as sleep). I know from long experience that Migration Assistant will replace installed preferences and containers with the original ones, if they differ. I would start Migration Assistant before another break. My time at the computer would be about fifteen minutes, which is less time than copying files to a new user and hunting down user files scattered throughout the user Library. None of this would be necessary if Apple supported reinstalling individual applications (iTunes, a critical app, is the exception.)