Dupe guide

Find duplicate files on an external drive on Mac

Clean up an external drive cluttered with duplicates. Safe, byte-exact deduplication that respects your backup data.

4 min read

External drives become graveyards. You back up a folder, then back it up again to a different subfolder, then a year later you copy “everything important” from your laptop onto the same drive. Now there’s 200 GB free where there should be 800, and you have no idea what’s a duplicate of what.

Two paths

The native way: Finder, drive by drive

You can dedupe an external drive manually in Finder, but it’s slow.

  1. Open the external drive in Finder.
  2. Switch to List view and sort by Size.
  3. Look for files with identical sizes — those are duplicate candidates.
  4. Use shasum in Terminal to verify pairs are actually identical: shasum "/Volumes/MyDrive/path/file1.zip" "/Volumes/MyDrive/path/file2.zip".
  5. Drag confirmed duplicates to the Trash.

This works but is painful for the volume of data on a typical external drive. And there’s a quirk: when you drag a file from an external drive to the Trash, macOS may permanently delete it instead of moving it to a recoverable Trash, depending on how the drive is formatted. That’s a real risk if you’re moving fast.

The Dupe way

Dupe treats external drives the same as internal ones, with one important difference: it handles Trash correctly across volumes.

  1. Plug in your external drive and let macOS mount it.
  2. Open Dupe and click “Add Folder.”
  3. Navigate to your external drive under “Locations” and select the top-level folder you want to scan (or a specific subfolder).
  4. Click “Scan.” Dupe hashes every file with SHA-256. External drives are slower than SSDs, so this may take a while for large drives.
  5. Review the duplicate groups. Dupe shows you the full path including the drive name so you can tell what’s where.
  6. Select copies to remove and click “Move to Trash.”

How Dupe handles external drive deletion safely:

Other safeguards that matter for external drives:

Dupe is $14.99 lifetime, no subscription. One scan of a cluttered external drive usually recovers tens of gigabytes.

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