Editing issue in Photos

Hey, Cat, your Dog is pretty cute-- and happy St Paddy's Day!

The problem is likely not with your hard drive or your OS. And stay away from disk cleaning apps-- they are known to mess up a Photos Library!

Does this happen only with RAW files?

It may be that your account has funny things going one, or your Library itself is wonky-- stuff like that. Here are some things to try that will tell us more:

• Restart the computer (of course-- and you've done that)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes. Safe Mode is different for different computers, so see this:

          Start up your Mac in safe mode - Apple Support 

• 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.
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. I'd use a copy of the Library, just in case. A large Library may take awhile, but in minutes, not hours. I just tried it with a 22GB Library and it took 2 minutes.
• Make a new user and see if the same problem recurs with it

Let us know what happens with your tests...

Cat, you cannot work with Photos while in Safe Mode. Starting up in Safe Mode is done to clean the caches and fix some issues, but you have to restart the Mac in normal mode, before you can use Photos again.

It is good to know, that a new library does not have the same issues, but this test is indicating that your current library seems to be damaged.

Safe Mode cannot fix the damages done to your Photos Library. If you have let Clean My Mac touch your Photos Library, it may have become damaged beyond repair. Did you allow Clean My Mac to "clean" the library? The early versions of CMM have removed the high resolution original RAW files as unnecessary "working" copies and replaced them by the edited versions, in many cases incorrectly, so Photos could no longer add additional adjustments to an image. The Library Repair tool can also not repair this kind of damage done by the early versions of CMM. It cannot put originals back that have been removed to save storage.

If you have been using Clean My Mac you may want to restore your Photos Library from a backup you may before your library became damaged. But do not overwrite your current library. You will need it to recover the new photos you added after the backup has been made.

Just to be sure, I would check your external drive as described here as well, so you can be reassured that it is working well: Check if a Mac disk is about to fail - Apple SupportApple SupportOfficial Apple Support › guide › mac-help › mac

You mentioned an upgraded hard drive.

It may also be a problem of the memory. Have you run a hardware check ? It might be a hardware problem with the memory or the graphics of your Mac. In that case it would not help to invest into third-party editors.

See: Use Apple Diagnostics to test your Mac - Apple Support