The solution for this was too insane to not share.
My iPhone recently went a few weeks without successfully completing a backup to Google Photos. Over that period I force quit the app almost daily to kick the backup, restarted the phone multiple times, and even applied the iOS 13.4 update, all without any change in behavior. The app would start the “Preparing backup” step and then never do anything. I had no idea how to fix it.
After weeks of messing with this I did some searching and ran across this lengthy Google forums post. Some of the recommendations suggest that flipping a toggle will immediately fix this, but that didn’t work for me so I’ll summarize the steps I took:
- In Google Photos go into Settings > Back up and sync, and toggle the two options for “Use cellular data…” to on.
- Now, turn off the Wi-Fi on your phone.
- Plug in your phone so it’s charging because this next step is going to kill the battery.
- In iOS Settings go to Display & Brightness > Auto-Lock and turn it to Never.
- Open Google Photos and tap your profile picture. The backup should kick off and enter the Preparing backup step, but this time your screen will stay on so you can monitor the progress.
- It will take awhile (maybe even a long while), but you should see the status eventually change to backing up a number of items.
Over the next 5-6 hours I saw the backup count go up and down multiple times (I have ~30k items) so don’t put too much stock in the number. At the end of it all I still had 2 failed items, but it finished! At this point I flipped Wi-Fi back on and the next backup completed those 2 failed items without issue.
Don’t forget to toggle the cellular backup back to off once you’re up and running! It’s been a few days with only Wi-Fi enabled again and everything is still working on my end.