Checksum copy vs Finder drag-and-drop: why Finder isn't a safe backup
Dragging files in Finder copies the bytes but never reads them back to confirm the copy matches the card. So a flipped bit, a dropped block or a truncated clip can land in your "backup" silently. Finder shows it as copied even when it's corrupt. A checksum copy hashes each file, copies it, then reads the copy back and hashes it again, proving every file is bit-for-bit identical before you wipe the card.
Copsy, a free Mac camera-offload app, gives you that verification for free using ASC-MHL, the film-set / streamer-delivery standard. Plug in a card, it copies to a drive (and optionally Google Drive or Dropbox), verifies every file, and pings you when it's safe to format.
Download Copsy for Mac → · macOS 13+ · no account needed
What a Finder copy actually does (and doesn't)
When you drag a folder of clips from a card to a drive in Finder, macOS reads the files from the card and writes them to the destination. That's it. There is no read-back step and no comparison of the source and the copy. Finder trusts that whatever it wrote landed correctly, marks the copy "done," and moves on.
For a Word doc that's fine, and if a small file were damaged you'd notice the moment you opened it. Footage is different: you usually don't review every frame of every clip before you reformat the card, and a single corrupt block in the middle of a 60 GB file is invisible until something tries to decode it. By then the original is gone.
What "no verification" really means
- No read-back: the copy is never read off the destination to check it survived the write.
- No hash comparison: nothing proves the copied bytes equal the source bytes.
- No record: there's no manifest you can re-check later or hand to a post house as proof.
- False confidence: a green "copy complete" looks identical whether the file is perfect or broken.
What a checksum copy does differently
A checksum (or "verified") copy adds the step Finder skips. It works in three moves:
- Hash the source. Each file on the card is read and run through a hash function (a checksum like xxHash, MD5, or SHA), producing a short fingerprint that's unique to those exact bytes.
- Copy the file. The bytes are written to the destination drive (and, with Copsy, to the cloud in parallel).
- Read it back and re-hash. The written copy is read off the destination and hashed again. If the two fingerprints match, the copy is proven identical down to the bit. If they don't, you get a hard error — not a silent bad file.
That read-back is what makes the difference. A copy counts as verified when the software goes back, reads the result, and confirms it. Watching itself write isn't enough. The fingerprints are also saved to a manifest so you (or anyone downstream) can re-verify the footage months later on a different machine.
How silent corruption sneaks in
"Silent" is the dangerous word: the failure leaves no obvious sign at copy time. Common causes during an offload:
- Failing media. A worn or counterfeit card, or a drive with bad sectors, returns or stores the wrong bytes.
- Flaky cables and readers. A cheap reader, a marginal USB-C cable or a loose connection can mangle data in transit.
- A bump mid-copy. A reader knocked loose or a card half-ejected can truncate a file or drop blocks.
- Power and software hiccups. A power blip, a sleep event, or a filesystem/buffer error can corrupt a write that still "completes."
- Bit rot later. Even a clean copy can degrade on the destination over time — which is why a saved checksum lets you re-verify down the road.
With a plain Finder copy, none of these raise a flag. You discover the damage weeks later when a clip won't play or your NLE throws a "media offline / corrupt" error — and the card was reused long ago. A checksum copy catches it while the original still exists, so you can recopy.
Finder copy vs checksum copy — side by side
| What happens | Checksum copy (Copsy) | Finder drag-and-drop |
|---|---|---|
| Copies the bytes | ✓ | ✓ |
| Reads the copy back off the destination | ✓ | ✗ |
| Compares source vs copy (hash match) | ✓ | ✗ |
| Catches silent corruption before you wipe the card | ✓ | ✗ |
| Saves a re-checkable manifest (ASC-MHL) | ✓ | ✗ |
| Copies to a drive AND cloud in parallel | ✓ | ✗ |
| Price | FREE | Built into macOS |
Finder is great for everyday files — this comparison is specifically about footage you can't reshoot. Last verified June 2026.
Where ASC-MHL fits in
ASC-MHL (Media Hash List) is the open, industry-standard format from the American Society of Cinematographers for recording the checksum of every file in an offload. It's the verification standard used on professional sets and for deliveries to studios and streamers, and it's portable: the manifest travels next to your footage so anyone, on any machine, can re-verify the copy later. That's the difference between "I'm pretty sure it copied" and a signed, auditable proof.
If you want the full breakdown of the format, hash types and how the manifest is structured, read our explainer: What is ASC-MHL? It covers why this is the right verification standard for any footage you care about, and why a verified copy means more than a Finder copy ever can.
Copsy: free verified copies on Mac
Copsy is a free Mac camera-offload app that replaces the risky drag-and-drop with a real, verified, ASC-MHL checksum copy — no signup, no card limit. Here's the workflow:
- Plug in the card. Copsy detects it by content and recognizes the source — Sony, ARRI, Blackmagic, RED, Canon, drones and audio recorders.
- Pick destinations. An external drive, plus optionally Google Drive or Dropbox — copied in parallel during the same offload.
- Copy, organize, verify. Copsy copies every file, organizes by date and source, records an ASC-MHL checksum on write, then reads each file back and confirms it matches.
- Get pinged. A Telegram message tells you it finished and verified — so you know it's safe to format the card.
The rule that saves footage
Never reformat a card until the copy has been read back and verified. A Finder "copy complete" is not verification. A matching checksum is. Pair that with a 3-2-1 backup and you've eliminated the two ways footage actually gets lost: a bad copy and a single point of failure.
Frequently asked questions
Stop dragging. Start verifying.
Copsy — a free Mac camera-offload app. Verified ASC-MHL copies to a drive and the cloud, so nothing is lost in silence.
⬇ Download Copsy for Mac