The question everyone asks on set
How do I know the copy is complete before I format the card?
The common answers — compare the file count, check the folder size, open a couple of clips — do not answer the question. Here is what does.
Why the usual checks prove nothing
| Common check | What it misses |
|---|---|
| The file count matches | A file that copied as garbage still counts as one file. |
| The folder size matches | A corrupted file has exactly the right size. So does a file full of zeroes. |
| I opened a few clips and they played | You checked the clips you opened. On a 400-clip card that is a sample, not a check — and damage tends to sit where nobody looked. |
| The Finder said it finished | The Finder reports that the write calls returned. It never reads anything back. |
The only thing that actually answers the question is reading every file back from the destination and comparing it with the source, byte for byte or by checksum. Everything else is a proxy for that, and every proxy has a way of being wrong.
Doing it for free, in the Terminal
You do not need to buy anything to verify a copy. If you are comfortable with a command line, this works today:
# copy, then verify by re-reading and comparing checksums
rsync -av --checksum /Volumes/CARD/ ~/Footage/ShootA/
# or check an existing copy against the card, without copying again
diff -r /Volumes/CARD/ ~/Footage/ShootA/Both read the data back and compare it. They are slower than a drag and drop, and that is the correct trade: the extra time is the verification. rsync --checksum is a genuinely reliable answer to this question and it costs nothing.
Two things the Terminal route will not give you
- A flushed device. If you verify straight after writing, the read can come out of the operating system's cache or the drive's own cache rather than the medium. See why that matters.
- A report. If a client, an insurer or a post house needs evidence that the copy was made and checked,
diffexiting quietly is not evidence.
Doing it with an application
Several macOS tools do verified offload, and some are free. Stow is free on macOS and Windows and writes an ASC-MHL manifest; lastcard is a one-time $59 for two Macs; ShotPut Pro and OffShoot are the long-established paid options at $169 (all checked 24/08/26). Card Offloader's own free tier does one verified local destination with no time limit.
Whatever you choose, these are the questions worth asking of it:
- Does it read the files back off the destination, or only checksum what it wrote?
- Does it bypass the cache when it reads back? If not, it may be comparing memory with memory.
- Is a failed file an error or a warning? Anything that can finish
with issues
can be misread as finished. - Can someone else check the result? A report only you can validate is a report that proves nothing to a client.
- Can it touch the card? Software that can format your source is software that can format your source by accident.
When it is actually safe to format
- Every file has been read back from the destination and matched. Not sampled — every file.
- You have more than one copy, on more than one physical device. Verification tells you the copy is good; it does not protect you from the drive failing next week.
- You have looked at the result yourself, rather than assuming the absence of an error message means success.
- Then format in the camera that will shoot the card, not on the computer.
And the corollary, which is the whole reason Card Offloader marks folders the way it does: if you are not sure, do not format. A card you have not wiped costs you an hour of inconvenience. A card you wiped after a bad copy costs you the shoot.