How do I move files to an external drive properly?

This is usually done to free up space, and it quietly converts files that had a backup into files that do not. The mechanics are simple; the thing worth getting right is what you are left with afterwards.

Safety

Step by step

1. Be clear whether this is archiving or backing up.Backing up means a second copy. Archiving means moving the only copy somewhere else. Archiving to a single drive leaves you more exposed than before, and that is worth deciding deliberately.
2. Check the drive's format.exFAT works on both Windows and macOS and is the right choice for a drive used with both. NTFS is Windows-native and read-only on a Mac by default; APFS is Mac-only. Reformatting erases the drive, so decide before you start.
3. Copy, do not move.Drag with copy rather than move, or use copy and paste. An interrupted move can leave files in neither location. The extra step of deleting later is cheap insurance.
4. Do it in batches by folder.One folder at a time rather than fifty gigabytes in one operation. It is easier to verify, easier to resume, and a failure costs you one folder rather than the whole transfer.
5. Leave it alone while it runs.Do not sleep the machine, close the lid, or unplug anything. Large transfers over USB take longer than the estimate suggests, particularly with many small files.
6. Verify before deleting anything.Compare the file count and total size, and actually open several files from the copy — particularly photos and documents. A folder that copied with the right size can still contain a corrupted file.
7. Delete the originals only after verifying.To the bin rather than permanently, and leave the bin unemptied for a week. This is the last safety net and it costs nothing.
8. Keep a second copy of anything irreplaceable.Photos, documents, records. One external drive is a single point of failure, and drives fail. A cloud copy or a second drive is what makes this safe.
9. Eject properly every time.Use eject or safely remove rather than pulling the cable. Writes are cached, and unplugging mid-write corrupts the file system rather than one file.
10. Label the drive and note what is on it.A physical label and a text file at the root listing the contents and the date. An unlabelled drive in a drawer becomes a drive nobody dares delete and nobody uses.
11. Store it somewhere different.A drive in the same room as the computer shares the same fire, flood and theft. If it is holding your only copy, somewhere else matters.
12. Check it every year.Plug it in, open a few files, and confirm it still mounts. Drives left unused for years frequently fail to spin up, and finding that out when you need them is too late.

Tips

• Many small files copy far more slowly than one large one. Zipping a folder of thousands of small files before copying is often much faster.
• SSDs are considerably more robust than mechanical drives for something carried around, and the price gap has narrowed.
• If the drive is for archiving photos, a cloud service alongside is worth the cost. Photographs are what people most regret losing.

Common mistakes

• Moving rather than copying — An interrupted move can leave files in neither place. Copy, verify, then delete — the sequence exists for a reason.
• Treating one external drive as a backup — Moving files off the computer means they now exist in exactly one place. That is an archive, and a fragile one — a backup means two copies.

If it doesn't work

Questions people ask