1. Check the destination before copying
Apple requires APFS or Mac OS Extended (Journaled) for the destination. It excludes a device used for Time Machine backups and advises against SD cards, USB flash drives, network storage and cloud storage for a Photos library. Use an appropriate directly connected external drive.
Our preparation check is to compare the library size with the destination’s available capacity and leave room for future imports. Do not erase a drive containing your only copy of other files just to change its format. A storage migration should start with a usable destination and a current backup, not a formatting experiment.
2. Copy the whole library and open the copy
Quit Photos. Locate the library in Finder; the default location is Pictures in your home folder. Drag the library to the external drive and wait for the copy to finish. Then double-click the library at its destination. Apple also supports holding Option while opening Photos to choose a library.
Our verification checklist is to inspect recent pictures, several older albums, edited images and a few videos. Check actual content rather than just the library’s filename. Leave the original in place during these checks so a failed or incomplete copy does not become your only working version.
3. Verify the System Photo Library and iCloud behavior
For iCloud Photos, the moved library must be the System Photo Library. Open Photos → Settings → General and use Use as System Photo Library if needed. Apple says a disabled button means the open library already has that role. Be careful when selecting a different library: enabling iCloud Photos merges its contents with iCloud and can trigger downloads.
If Optimize Mac Storage was enabled, Apple says full-size originals can be in iCloud while smaller versions remain locally. Our consequence check is that copying the local library does not prove every original is available offline. Decide whether you require originals on the external drive and verify that requirement before calling the migration complete.
4. Check what your backup actually includes
Apple recommends a local library backup even with iCloud Photos. Referenced files stored outside the library are not included when the library is backed up, so those need separate coverage. Apple’s referenced-file guide explains how to locate them and consolidate them if appropriate.
Our backup check is to identify a recovery copy on a different device from the working library. After you remove the Mac’s original, the external library becomes your working copy; it is no longer an extra copy. Verify that your backup routine covers this new location rather than assuming the old arrangement follows it automatically.
5. Remove the original only after verification
Once the destination and backup are verified, quit Photos and move only the original library to Trash. Review before emptying it. Connect the external drive before opening Photos: Apple warns that an unavailable destination can cause Photos to create an empty library in the default location.
If you later see an empty library, our first check is the drive connection and the selected library, not an assumption that the photos vanished. Quit Photos and use the Option-key library chooser to inspect the intended destination. Resolve copy, permissions or missing-content errors before permanently removing anything else.
References & further reading
These guides combine the linked official guidance with ClearDisk’s documented behavior. Your files and app settings determine the right next step.