Downgraded from Big Sur to Mojave on Macbook Air, Photos app won't open

On my Macbook Air, I downgraded from Big Sur to Mojave, and the Photos app won't open. Photos are located on an SD card and not on the MacBook HD. The reason I downgraded was because the essential software I constantly used did not work on Big Sur.

I simply restored from a backup I had made on Mojave before I got the local Mac service people to install a new (larger) hard drive. It may be (I don't recall) I opened the Photos app when Big Sur was installed, and if so, it may have altered the database setup.

It's not truly surprising. Apple frequently makes format/database changes with upgrades. The resulting files are almost never convertible 'downhill', there is simply no provision to be able to do that.

There appears to be a 'get out of jail free' card for Photos, though, using iCloud as an intermediary. There is a long [& outdated] explanation on Downgrading a Photos Library to a Previous Version of Photos with the Help of iCloud Photos which I have never tested, so cannot comment on whether it will still work from Big Sur right the way back to Mojave.

I can confirm that deleting the Big Sur Library, creating a new Photos Lib, making it your System Library and turning on iCloud Photos will download a new Mojave compatible library.

This does rather emphasise the need for a solid backup strategy, to prevent issues like this in future. Personally, not only do I keep backups of everything, but before I do any major OS upgrade, I clone my entire boot drive as a 'belt & braces' for if I decide to go back. There are frequently issues trying to go 'downhill' using Time Machine, so I consider this to be a vital step.

I believe I found the answer. Go to Finder, then Pictures, then right click Photos library and select Show Package Contents. Select Originals folder and drag to left column of Finder under Pictures or wherever in that column. Open folder and all photos are in numbered folders. Then go to App Store and located in your account there should be the iPhoto app, download and boot it, then under File menu select "Import to Library' and select the 'Original' folder mentioned above and all your photos/videos load into the iPhoto app. I tried doing the same in the "Photos' app but it didn't work. iPhoto will do for me for the moment, the only difference "Photo" app syncs your pics with other Apple devices, which I'm not interested in. But at least I was able to restore all my inaccessable photos and I'm back in business on my Mac. Possibly when I upgrade to next macOS Photos app will be able to access my pics also.