Photos is Apple's photo-management app that recently replaced iPhoto with the release of OS X Yosemite 10.10.3. It certainly gets the job done, but it can still be prone to errors once in a while. The Library folder contains many of the resources that installed applications need to use, including preferences, support documents, plug-in folders, and the files that describe the saved state of applications. It has long been a go-to location for troubleshooting issues with individual applications or components shared by multiple applications. Added security, clear photos and fancy features are some of these reasons. Though this sounds a substantial move, the majority are hit by ‘macOS needs to repair your library to run applications’ message just after the update. The message pops up each time you open an application in your mac. Fill your library, not your device. ICloud Photos can help you make the most of the space on your Mac. When you choose “Optimize Mac Storage,” all your full‑resolution photos and videos are stored in iCloud in their original formats, with storage-saving versions kept on your Mac as space is needed.

It’s easy from the perch of writing the Mac 911 column to appear as if I have all the answer. Dear readers, I do not. I am always delighted to research on your behalf, but sometimes problems fall into my lap that I know will help you all, because I’m unable to find a ready solution and nobody in any forum or on any blog has found an answer, sometimes across many years.

After making a fresh backup of your photo library, follow the troubleshooting steps below to fix your missing or incorrect thumbnails. Let us know in the comments which step works for you. Use the Photos Library Repair Tool on a Mac. In macOS, the Photos app features a Library Repair Tool which can fix incorrect thumbnails.

Photos for macOS is a bugbear for many of you, one of the most regular sources of questions. But I encountered a sync problem that no one has written in about, yet I can find a history of frustrated people across the Internet trying to solve it. I did find a solution—but you might not like it!

My problem emerged a few weeks ago, when I noticed that Photos for macOS on my office computer, an iMac running Mojave, was out of date. I checked my laptop Mac, my iPhone, and iCloud.com: photos were synced and up to date in each of those locations.

This has happened before, and I go through a series of escalating troubleshooting steps:

Quit Photos and re-launch it.
Using the Terminal to “kill” certain Photos-related background agents and jobs. (This is a little tweaky and can cause problems, so I hesitate to recommend it. In any case, it didn’t help.)
Restart my Mac.
Rebuild the Photos database by holding down Command-Option while launching the app and following prompts.
Disabling iCloud Photos and re-enabling it in Photos > Preferences > iCloud. This can force a re-sync, though often it’s relatively fast as Photos and iCloud seem to bypass pictures that are in both places.

Mac Photos App Repair Library Mac

None of this worked. After reading years’ worth of posts of people trying to overcome the problem, I tried one suggestion: deleting a deeply nested folder in the Photos library that tracks iCloud synchronization.

Warning! Apple doesn’t advise making these kinds of low-level changes. I strongly recommend making a full backup of your Photos library, if not a fresh full clone of your drive or forcing a Time Machine update, before proceeding.

Find your Photos library and Control-click it, and then select Show Package Contents. Now traverse down to:

private/com.apple.cloudphotosd/CloudSync.noindex

With Photos not running, I deleted the contents of this folder, which required entering an administrative password. I then-relaunched Photos.

Photos and iCloud apparently had a behind-the-scenes confab and decided my copy of Photos had never synced with iCloud at all. This led Photos on my Mac to re-upload about 45,000 images and videos, even though they were identical in my library at iCloud.com.

Fortunately, I have gigabit Internet service with no cap on usage and no overage fee. (The local legacy wired telephone company has rolled out fiber like mad and charges a relatively low amount for it—even less than slower service via a cable ISP.)

If you have more normal broadband service, even in the 50Mbps to 100Mbps range, or get throttled, blocked, or charged overage fees above a certain amount of monthly data, this won’t seem like a good solution to you at all. (Although during the pandemic, a number of ISPs with home broadband service have removed caps and overage charges, including AT&T and Comcast.)

The resync took about two days and showed steady progress. At the end, all my media remained in exactly the same state, but my Mac’s Photos library was finally up to date with other devices in its iCloud account set.

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How To Repair Mac Photos Library

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Mac Photos App Repair Library

Glenn Fleishman is the author of dozens of books. His most recent include Take Control of Your M-Series Mac, Take Control of Securing Your Mac, Take Control of Zoom, and Six Centuries of Type and Printing. In his spare time, he makes Tiny Type Museums. He’s a senior contributor to Macworld, where he writes Mac 911.