Bredo / Guides / Before you erase a card

How to check a camera card copied correctly before erasing it

Take a checksum of every file on the card, take the same checksum of every file in the copy by reading it back off the destination drive, and compare the two lists. If every file matches and the footage exists in two places on two physical disks, the card is safe to format. If you have not done that comparison, all you know is that a copy operation did not report an error, which is a much weaker claim than it sounds.

Why "the copy finished" is not the same as "the copy is good"

When the Finder's progress bar reaches the end it is telling you that it handed every byte to the operating system without being told no. It has not gone back to look. A copy can be wrong in ways that produce no error at all: a USB cable that drops packets under heat, a drive with a sector going bad, a card reader on a loose hub, a card pulled half a second early while data was still in a write buffer.

The result is a file at the destination with the right name, usually the right size, and a handful of corrupt bytes somewhere in the middle. It will very likely still open. The damage shows up in the edit, three weeks later, at the one moment in the interview that mattered, long after the card was reused.

Checking a card copy, step by step

1. Lock the card and leave it alone. Slide the write-protect tab on an SD card, or set the camera aside without shooting on it again. Nothing should write to the card between the copy and the moment you are satisfied.
2. Copy the whole card, not the clips you think you want. Copy the entire folder structure, including sidecars, proxy folders and the camera's own database files. Formats such as RED RDC, Panasonic P2 and Canon XF fall apart when files are lifted out of their folders.
3. Take a checksum of every file on the card. Compute a hash for each file by reading it off the card, not from anything cached in memory. MD5, xxHash64 and SHA-1 are all fine for catching a bad copy; xxHash64 is much faster and no less useful here.
4. Read the copy back and compare. Compute the same hash for every file at the destination, reading each file from the drive, and compare the two lists. A file whose hash differs, or that is missing, means the copy failed and must be made again.
5. Make the second copy before you erase anything. Repeat onto a second drive on a separate physical disk and check that too. A card is safe to erase only once its contents exist and have been checked in two places.
6. Format in the camera, not on the Mac. Once both copies are checked, format the card in the camera that will use it, so the camera writes the folder structure and file system it expects.

Doing it by hand in Terminal

You do not need software for this. From the card, write a list of hashes, then produce the same list at the destination and compare them:

cd /Volumes/A7IV_CARD
find . -type f -not -name ".*" -exec shasum -a 1 {} \; | sort > ~/card.txt

cd ~/Footage/2026/2026-09-14\ Harbour\ walk/A7IV_A001
find . -type f -not -name ".*" -exec shasum -a 1 {} \; | sort > ~/copy.txt

diff ~/card.txt ~/copy.txt && echo "every file matches"

It is slow, because every byte is read twice, and that is the point. Purging the disk cache between the two reads is wise on a Mac with plenty of memory, otherwise a small file can be hashed from RAM rather than from the drive you are trying to test.

What a checksum cannot tell you

A matching hash proves the copy is identical to what was on the card. It does not prove the footage is good. If the camera wrote a corrupt frame, or the card was already failing while recording, the damage is in the original and your copy reproduces it faithfully. This is why spot-checking playback still matters: scrub a few clips, especially the first and last of each card, and watch the takes that cannot be repeated.

Nor does a check say anything about tomorrow. A drive that was fine an hour ago can fail tonight. The check tells you that the copy was right at the moment it was made, which is exactly the thing you need to know before destroying the original, and nothing more.

Keep the hash list with the footage

Once you have a list of checksums, do not throw it away. Written as an ASC MHL file in an ascmhl/ folder next to the footage, it is a standard record that other tools can read, and it lets you re-check the same files years later to find out whether a drive has quietly rotted. Without it you can compare a drive against another drive, but you cannot compare either against what the camera originally wrote.

For the wider routine this check sits inside, see the one-person offload workflow. For keeping those records straight across a shelf of drives, cataloguing external drives covers where the hash lists should live.

Questions

Does the Finder tell me whether a card copied correctly?

No. The Finder reports that it finished writing, which is not the same as reading the result back and finding it identical. Copies can fail quietly through a failing cable, a drive with a bad sector or a card pulled a moment early, and the file will still appear at the destination with the right name and often the right size.

Is it safe to erase a card once one checked copy exists?

No. Until the second copy exists, the card is one of your two copies, and erasing it takes you to one. Erase only when the footage has been checked in two places on two physical disks, and keep the card unformatted for as long as you reasonably can.