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How to test a memory card for fakes & errors on Mac

Written by a working DIT · Copsy · Last verified June 2026

Short answer

To test a memory card for fakes on a Mac, run a full write-then-read-back capacity test (with a tool like F3) that fills the entire card with known data and reads it all back — a fake card silently corrupts everything written past its real size, so anything that doesn't read back intact proves the card is lying about its capacity.

Counterfeit reduced-capacity cards report a big size but only store a fraction of it. Test every new card fully before a shoot, and treat any checksum mismatch during an offload as a red flag.

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Why fake and failing cards are so dangerous

The scary thing about a bad memory card isn't that it dies loudly — it's that it lies quietly. A counterfeit reduced-capacity card is a small chip flashed to report a big size. The Mac sees "256 GB," the camera sees "256 GB," and recording looks completely normal. But the card might only have 32 GB of real storage. Everything you write past that real limit is silently dropped or wrapped around and overwrites earlier data.

The result: clips that played back fine on the monitor turn out corrupt, truncated, or missing the moment you try to read them on the offload. By then the shoot is over and the card has been reused. A failing (genuine but worn or damaged) card behaves the same way in practice — bad sectors return garbage instead of the bytes you wrote. Both problems are invisible until something reads the card back and checks it.

The one test that catches a fake: full write-then-read-back

One-line answer: fill the whole card with known data, then read it all back — if every byte returns intact, the card is real; if anything past a certain point is corrupt, the card is fake or failing.

This is the only test that actually proves capacity, because it's the only one that forces the card to store data across its entire claimed size. Checking "Get Info" in Finder or reading the label tells you nothing — that's exactly the number a fake is faking.

Back up anything on the card first. A full-capacity test is destructive — it writes across the whole card. Only test a card that's empty or already offloaded.
Install F3 on your Mac. F3 ("Fight Flash Fraud") is the standard open-source capacity tester. Install it with Homebrew: brew install f3. It gives you two commands, f3write and f3read.
Write test data across the whole card. Point f3write at the mounted card (e.g. f3write /Volumes/UNTITLED). It fills every gigabyte with known patterns. Let it finish — this writes the full claimed capacity, so it takes a while on big cards.
Read it all back and check. Run f3read /Volumes/UNTITLED. On a genuine card every sector reads back as written. On a fake, F3 reports corrupted or overwritten data past the card's real size — that's the proof it's lying about capacity.
Reformat in your camera before use. Once a real card passes, format it in-camera so it's clean and aligned for recording.

If you'd rather not touch the command line, there are GUI capacity testers built on the same write-and-verify idea (and Windows-only options like H2testw exist, but on a Mac F3 is the go-to). The principle is identical: full write, full read-back, compare.

Sanity-check speed too — fake cards often fake their speed class

Capacity isn't the only thing counterfeiters lie about. A card rated for high-bitrate recording that can't actually sustain its write speed will drop frames or stop recording mid-take. After a capacity test passes, run a quick speed benchmark with AmorphousDiskMark (free on the Mac App Store) and compare the measured sequential read/write to the card's printed speed class. If a "V90 / 300 MB/s" card benchmarks at 40 MB/s, it's either fake, worn, or being throttled by a slow reader — none of which you want on a job.

Buy from reputable sellers in the first place

The cheapest fix is not buying a fake. Counterfeits flood marketplaces through third-party sellers, so buy memory cards directly from the manufacturer or an authorized retailer, not from an unknown seller offering a flagship card at half price. If the price looks too good, the capacity is usually the catch. Keep the receipt, and still run a full capacity test on arrival — even legit channels occasionally ship a bad unit.

How a verified offload catches a card that's lying

Copsy is a safety net, not a card tester

Copsy doesn't run a capacity test — for that, use F3 before a shoot. But Copsy verifies every file it copies with ASC-MHL checksums during the offload. If a card is fake or failing, the bytes that read back won't match what was recorded, the checksum won't match, and Copsy flags the mismatch instead of letting a silently-corrupt file pass as a good copy.

So even if a bad card slips through onto a shoot, the verified offload is where it gets caught — at the desk, with the card still in hand, not weeks later in the edit. Test new cards up front; let Copsy back you up on every card after that.

How Copsy offloads and verifies a card

Plug in the card. Copsy detects it by content and recognizes the camera/source (Sony, ARRI, RED, Blackmagic, Canon, drones, audio recorders).
Pick destinations. An external drive, plus optionally Google Drive or Dropbox — copied in parallel (one cloud at a time).
Walk away. Copsy copies, organizes by date and source, and verifies every file with ASC-MHL checksums.
Get pinged. A Telegram message tells you it finished and verified — and if a file fails verification, that's your bad-card warning.

Frequently asked questions

How can I tell if an SD card is fake?

A fake card almost always reports a larger capacity than it really has, so the giveaway is a full write-then-read-back test. Fill the entire card with known test data and read it all back: a genuine card returns every file intact, while a fake starts returning corrupt or zeroed data once you write past its real size. Suspiciously low prices, off-brand packaging and capacities that don't match the seller's claims are warning signs, but only a full-capacity test on a Mac proves it.

How do I test a memory card on a Mac?

Use a full write-and-verify capacity tool such as F3 (f3write/f3read), installable via Homebrew, to write test data across the entire card and read it back so you catch fake capacity and bad sectors. Then run a speed tool like AmorphousDiskMark to confirm the card's real read and write speeds match its rating. Always test a new card fully before you trust it on a paid shoot.

Can a fake SD card corrupt my footage?

Yes. A counterfeit reduced-capacity card reports a big size but only has a fraction of that real storage. Anything written past the real limit is silently dropped or overwritten, so clips look fine in-camera but turn out corrupt or missing when you offload. That's why fakes are so dangerous on a shoot — the failure is invisible until you try to read the files back.

How does Copsy help catch a bad card?

Copsy isn't a dedicated card tester, but it verifies every file it copies with ASC-MHL checksums during the offload. If a card is fake or failing, the corrupt files won't match their checksums and Copsy flags the mismatch instead of letting a bad copy pass silently. It's a complementary safety net that surfaces a lying card the moment you offload, on top of testing new cards before a shoot.

Is Copsy free?

Yes, Copsy is free to download and use today, with no card limit and no account required. It runs on macOS 13 or later.

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