How to Find Duplicate Files on an External Hard Drive (Mac)

To find duplicate files on an external hard drive, connect it and add it to a duplicate finder: our free duplicate file finder runs in your browser and can compare several drives at once, and Terminal can do it with one command. The harder part comes after the list. Some of those duplicates are wasted space, and some of them are your backups.

If you shoot for a living, you probably have the same footage on three drives on purpose. Deleting "duplicates" across drives without knowing which copies are backups is how people end up with one copy of a wedding, on one drive.

First: Which Duplicates Are Actually a Problem?

Duplicate finders group identical files. They don't know why the copies exist. Before deleting anything, sort each group into one of three cases:

The goal for anything you care about is two or more copies on different drives, and zero extra copies on the same drive. A duplicate finder gives you the raw list; this table is how you read it.

Method 1: The Free Browser Tool (Several Drives at Once)

Our duplicate file finder for Mac runs entirely in your browser. Nothing is installed and nothing is uploaded; it keeps working with the internet turned off.

1. Connect the external drive (or drives).
2. Open the tool and add the first drive as a folder. Chrome, Edge, Arc, and Brave can open a mounted drive directly with the system folder picker; Safari and Firefox use a standard folder chooser.
3. Add the second drive, and your Mac's folders if you want. The list updates with each one, so copies on different drives land in the same group.
4. Review the groups. Files with the same name and size are listed as duplicates without reading their contents, which makes even a 100GB video instant. Files with the same size and date but different names (a renamed export, for example) are listed as possible duplicates, and you can run a checksum on those to confirm.
5. Export the list as CSV if you want a record of the paths.

The tool doesn't delete anything and forgets the drives when you close the tab. That's on purpose: deleting straight from a duplicate list, without checking which copy is good, is how people lose the only intact file.

Method 2: Terminal (Free, Reads Every File)

This command hashes every file over 100KB on the drives you give it and prints groups of files with identical contents, whatever their names. Replace the drive names with yours:

find /Volumes/DriveA /Volumes/DriveB -type f ! -name '.DS_Store' ! -name '._*' -size +100k -print0 \
  | xargs -0 shasum -a 256 \
  | sort \
  | awk '{h=$1; sub(/^[^ ]+  /,""); if (h==prev) {if (!shown) print "\n" prevf; print $0; shown=1} else shown=0; prev=h; prevf=$0}'

I tested it on macOS 15.5 with two test drives. It found all three kinds of duplicate:

DriveA/2024/Lisbon/A002.MP4
DriveB/Exports/lisbon-final copy.MP4

DriveB/Archive/A003-2.MP4
DriveB/Archive/A003.MP4

DriveA/2024/Lisbon/A001.MP4
DriveB/Archive/A001.MP4

The first group is a renamed copy (a name-based finder would miss it), the second is two copies on the same drive (waste), and the third is the same clip on two drives (a backup).

The catch is speed. It reads every byte, so it runs at the speed of the drive: roughly 1 to 2 hours per TB on a desktop hard drive. The browser tool is faster for a first pass because it only hashes the files that need it.

Method 3: Duplicate Finder Apps

If you want an app that can also delete:

• dupeGuru is free and open source, with a Mac version. It scans folders or whole drives, matches by contents, and can move or delete duplicates. It's the one most Reddit threads recommend.
• Gemini 2 (MacPaw) and Duplicate File Finder (Nektony) are paid, polished apps aimed at clearing space on your Mac, with previews and one-click cleanup.

All of them can scan an external drive. None of them knows which of your drives is the backup, so the table above still applies before you click delete.

What About Duplicate Photos?

If the photos are in Apple's Photos app, use Photos itself. On macOS 13 Ventura and later, Photos shows a Duplicates album in the sidebar under Utilities, and you can merge each set, keeping the best-quality version and its metadata. Apple's steps are in Remove duplicate photos and videos. Don't run a file-level duplicate finder inside a Photos library package; deleting files in there corrupts the library.

For RAW files managed in Lightroom Classic, delete duplicates from inside Lightroom (Remove, then Delete from Disk), so the catalog stays in sync. Deleting them in Finder leaves Lightroom showing missing photos; our Lightroom external drive guide covers how to fix that.

Before You Delete: A Short Checklist

1. Open the copy you're keeping. Play the video, open the RAW. A file with the right name and size can still be damaged.
2. Confirm it's identical, not just similar. Use a checksum (the browser tool's checksum step or shasum -a 256 on both files). Same name and size is not proof.
3. Count the copies on other drives. If the only other copy is the one you're about to delete, you're deleting a backup.
4. Move to the Trash first, and empty it a week later.

Why Duplicates Pile Up Across Drives

Mostly from offloading the same card twice ("did I copy this already?"), copying a project to a new drive and never cleaning the old one, exporting to several places, and drag-copying instead of moving. It's normal, and it's a symptom of not knowing what's already where. Our guide to keeping track of files across external drives covers the habit side of that.

Tusk tackles the cause instead of the symptom. Point it at a drive you already have and it hashes every file with BLAKE3, keeps that index after you unplug the drive, and knows when two drives hold the same file. Backups become intentional copies it tracks per destination, so a second copy on another drive shows up as a backup, not a mystery duplicate. Once a verified copy sits in a project, Tusk can move the extra copies to the Trash. It's Mac only, $49 for the first 100 licenses, $79 after, one-time, with a 14-day trial.