Move Your Photos Library to an External Drive on Mac

A full-resolution Photos library is often the single biggest item on a Mac — libraries over 100 GB are common. macOS is happy to run Photos from an external drive instead, which frees your internal SSD without losing a single picture. Here's how to move it safely and keep iCloud Photos syncing.

Before you move: check the drive

Three things decide whether this goes smoothly:

• Format. The drive must be APFS or Mac OS Extended (Journaled). Photos won't work reliably on exFAT or FAT32 volumes because they can't store the permissions the library needs. Check the format in Disk Utility — and remember that reformatting erases the drive, so do it before putting anything else on it.
• Space. You need room for the whole library plus headroom. Select Photos Library.photoslibrary in your Pictures folder and press ⌘I (Get Info) to see its size, or use Terminal:

# Check the library size (read-only, changes nothing)
du -sh ~/Pictures/Photos\ Library.photoslibrary

• Availability. Photos needs the drive connected every time you open it. A desk drive that stays plugged in beats a pocket SSD you carry around. Avoid putting the library on your Time Machine backup drive — mixing the two makes backups and space management messy.

Copy the library to the external drive

1. Quit Photos completely (⌘Q). Never copy the library while Photos is open.
2. In Finder, open your Pictures folder (in the Go menu, choose Home, then open Pictures).
3. Drag Photos Library.photoslibrary onto the external drive. Because you're dragging across volumes, Finder copies it — the original stays put, which is exactly what you want as a safety net.
4. Wait for the copy to finish completely. For a 100 GB library this can take a while, especially over USB; don't sleep the Mac mid-copy.
5. Hold Option and launch Photos. In the library chooser, click Other Library…, select the copy on the external drive, and click Choose Library.

Photos now opens from the external drive. Everything — albums, edits, metadata — comes with it, because the library file contains all of it.

Keep iCloud Photos working

iCloud Photos syncs only the System Photo Library, and that designation didn't move with the copy. To transfer it:

1. With the new library open, choose Photos → Settings (⌘,) and open the General tab.
2. Click Use as System Photo Library. If the button is greyed out, this library is already the system library.
3. Switch to the iCloud tab and confirm iCloud Photos is on. Photos will compare with iCloud and pick up syncing — it should not re-upload everything.

Only one library can be the System Photo Library at a time, so once you flip it, the old copy on the internal disk becomes a plain local library.

Delete the old library — but not yet

The point of all this is reclaiming internal space, but verify first. Browse the new library, check the photo and video counts against the old one, and open a few recent shots at full resolution. Use it normally for a week or two with the old library untouched.

When you're confident, move the old Photos Library.photoslibrary from Pictures to the Trash and empty it. That's the undo point gone, so this is the step to be slow about. If free space doesn't jump immediately, give macOS a few minutes — the space is often counted as purgeable before it's released.

Shrink it before you move it

If the copy estimate looks painful, slim the library first — clear the Duplicates album, empty Recently Deleted, and hunt down giant videos. Our guide to reducing your Photos library size covers all three, and a smaller library copies (and backs up) much faster.