Moving files to an external drive without breaking anything
Apple's advice is one line: move media to an external drive. Here is which things move safely, which must never be moved, and how to move a Photos library.
Every guide to a full Mac says the same thing: move your media to an external drive. It is good advice and it is where the instructions usually stop, which is a problem, because the difference between moving a folder and moving a library is the difference between a tidy Mac and an app that will not open.
What moves safely, and what does not
| Safe to move? | |
|---|---|
| Video files, exports, archives, installers | Yes. Ordinary files, nothing tracks them |
| Old project folders you are finished with | Yes |
| The Photos library | Yes, but through Photos, not by dragging alone |
| The Music library | Yes, same caveat |
| Anything in ~/Library | No. Apps expect it where it is |
| Applications | No. Move the ones you do not use to the Trash instead |
| Anything a running app is writing to | No |
Format the drive correctly first
In Disk Utility, choose APFS for a drive that will only ever be used with Macs. It handles large files properly and is what macOS expects.
Choose exFAT only if the drive genuinely has to be read by Windows too. It has no journaling, which means an unplug at the wrong moment can lose more than the file being written. Do not put a Photos library on exFAT.
Moving a Photos library, which is the one people get wrong
- Quit Photos.
- Drag
Photos Library.photoslibraryfrom Pictures to the external drive. Wait for it to finish completely: it is one bundle containing tens of thousands of files, and it is slow. - Hold Option and launch Photos. It asks which library to use.
- Choose the copy on the external drive, and confirm the photos are all there.
- If you use iCloud Photos, open Photos, Settings, General, and click Use as System Photo Library.
- Only then move the original to the Trash.
The order matters. Deleting the original before Photos has opened the copy leaves you with a library nothing has verified. For what is inside that bundle and why it is so large, see what the Photos library is really holding.
Two things that are now true and were not before
- The drive has to be plugged in. A library on an external drive is unavailable without it, and iCloud Photos will not sync while it is missing.
- That drive is now a single copy. Moving is not backing up. Whatever you moved is one drive failure from being gone, so it needs to be in your backup rota from the day it moves.
That second point is the one that turns a tidy Mac into a lost photo library a year later. If Time Machine is your backup, check the external is not in its exclusion list: see how Time Machine uses your disks.
Before you buy a drive at all
Moving files is worth doing when the files are genuinely yours and genuinely large. It is the wrong answer when the space is going to caches, build output or old device backups, none of which are worth keeping anywhere. Those regenerate or are already redundant, and deleting them costs nothing.
Check what is actually large first. The safest order to free 100 GB works through it from the safest end, and on most machines it reaches a hundred gigabytes before a drive is needed at all.
Common questions
How do I move my Photos library to an external drive?
Quit Photos, copy the library bundle to the drive, then hold Option while launching Photos and choose the copy on the external. If you use iCloud Photos, set it as the System Photo Library. Only delete the original once the copy has opened and been checked.
Should I format an external drive as APFS or exFAT?
APFS for a Mac-only drive: it handles large files properly and is what macOS expects. exFAT only if Windows genuinely has to read the drive, and never for a Photos library, because it has no journaling and an unplug at the wrong moment can lose more than the file being written.
Is moving files to an external drive the same as backing them up?
No, and confusing the two is how libraries get lost. Once a file lives only on the external drive, it is a single copy on a device that can fail. Anything you move needs to be included in your backups from the day it moves.