Move Photos library to external drive and maintain Photos app access on iMac

How to Move Your Photos Library to an External SSD on an iMac (macOS 10.13 High Sierra)

You can move your entire Photos library to an external SSD and continue using it in the Mac Photos app just as you do now. The main difference is that your photos will be stored on the SSD instead of your iMac’s internal drive.

Follow the steps below to move your library safely while keeping your photos, albums, edits, and organization intact.

Before You Begin

Make sure you have the following in place:

• Enough space: Your external SSD should have sufficient capacity for the entire Photos library, with some room left for future photos.
• Correct format: For a drive dedicated to your Mac, Mac OS Extended (Journaled) is a good choice for High Sierra. APFS is also supported.
• A backup: Keep a separate backup of your library until you have confirmed that everything works correctly on the SSD.
• A reliable connection: The SSD must be connected and mounted whenever you want to access your library.

Important: Reformatting a drive erases its contents, so back up any existing files before doing so.

Step 1: Find Your Photos Library

1. Open Finder.
2. Select Go >> Home from the menu bar.
3. Open the Pictures folder.
4. Look for the file named Photos Library.photoslibrary.

This is your complete Photos library package. It contains your photos, videos, albums, edits, and other library data. You need to move the entire package, not individual pictures.

Step 2: Copy the Library to Your SSD

1. Open Photos and select Photos >> Quit Photos to close the app completely.
2. In Finder, drag Photos Library.photoslibrary from the Pictures folder to your external SSD.
3. Wait for the copy to finish. Depending on the size of your library, this could take some time.

For now, leave the original library on your iMac. It's best to confirm that the SSD copy works before deleting anything.

Step 3: Open the Library from the SSD

1. Make sure the SSD is connected.
2. Open the SSD in Finder.
3. Double-click Photos Library.photoslibrary.

Photos should open using the library stored on the external drive.

If Photos opens a different library, quit the app. Then hold down the Option (⌥) key while launching Photos. Select the library on the SSD and click Choose Library.

Step 4: Set the SSD Library as Your Main Library

This step tells Photos which library to use by default.

1. With the external library open, select Photos >> Preferences.
2. Click the General tab.
3. Click Use as System Photo Library.

If the button is greyed out, the library may already be set as the System Photo Library.

If you use iCloud Photos, this setting is particularly important. Check the iCloud tab in Preferences afterward to make sure your existing settings are still correct.

Step 5: Check Everything Before Removing the Original

Before deleting the library from your iMac, take a few minutes to confirm that everything is working:

• Open several photos, including older ones.
• Play a few videos.
• Check your albums and edited photos.
• Confirm that the library appears complete.
• Quit Photos, disconnect and reconnect the SSD, and open the external library again.

Once you're satisfied that everything works and you have a separate backup, you can delete the original library from your iMac's Pictures folder to free up space.

What Happens When You Disconnect the SSD?

Situation

What to expect

SSD connected

Photos can access your library normally.

SSD disconnected

Photos cannot access the library stored on the SSD.

Photos opened without the SSD

Photos may open or create a different, empty library on your iMac. Your original library remains on the SSD.

SSD reconnected

Open the external library if Photos doesn't select it automatically.

My recommendation: Leave the Photos app installed on your iMac and move only the Photos library to the SSD.

This gives you the extra storage space you want without changing how you normally use Photos.

For additional guidance, see Apple's article: Move your Photos library to save space on your Mac.

If you are using iCloud Photos, and you move your "system" Photo library to an external drive, I would suggest that you keep that drive attached and mounted as much of the time as reasonably possible.

To the extent that synchronization takes place in the background, it follows that background processes would only have a chance to do synchronization work when the "system" library was accessible.

Owl-53 wrote:

How to Move Your Photos Library to an External SSD on an iMac (macOS 10.13 High Sierra)

…

If you use iCloud Photos, this [System Photo Library] setting is particularly important.

FYI, Apple recently increased the system requirement for iCloud Photos to macOS 10.14 (Mojave) or later.

System requirements for iCloud - Apple Support

The procedure you describe is generally applicable to many versions of macOS, but if someone has been using iCloud Photos successfully under High Sierra, and is now running into difficulties doing so, this quiet increase in iCloud system requirements may be the cause.

Thank you everyone, this is really helpful.

My reason for doing this.

I am copying/moving my entire media/photos and iTunes from my iMac to an external SSD as Apple have updated my iPhone to ios27 and my iMac, which is running iTunes and OS 10.13 is not supported. This means the iMac does not recognise the iPhone when connected by cable so I cannot back it up or change the media on the iPhone. So far Apple have not supplied a software patch to enable my old IMac to function as it used to. Solution is buy a used MacBook Air laptop with M1 chip and use that for iPhone storage and media manipulation. Hence all the media files to be stored in an external ssd connected to the Mac Book Air. The laptop has insufficient hard disc space for the all media files.

Thank you. Not unlike the method used to move my iTunes library onto an external SSD to free up space.

You are correct 👍

Happy computing

🦉-53