Dragging a camera card into a folder in Finder copies the files and tells you nothing about whether they arrived intact. For a holiday snap that is fine. For a paid shoot, on a card you are about to format, it is a gamble you take without noticing.
What Finder does not tell you
A Finder copy reports success when the write completes. It does not read the files back afterwards to confirm they match the originals. Most of the time they do. Occasionally they do not, because of a flaky reader, a marginal cable, a card near the end of its life, or a write interrupted by a sleeping laptop.
The failure is quiet. You get a file of the right size with the right name that will not open properly, or a clip with corruption partway through. You usually discover it much later, by which point the card has been formatted and reused several times over.
What a verified offload does instead
A verified copy adds one step: after writing, it reads both the source and the destination and compares a checksum of each. A checksum is a short fingerprint calculated from the actual bytes, so two files match only if their contents are identical, not merely their name and size.
If a byte went astray, the fingerprints differ and you are told before the card is cleared. That is the entire point. Verification is not about being cautious, it is about finding out while the original still exists.
The rule worth adopting
Do not format a card until the copy has been verified and a second copy exists.
The card is your only original until the moment there are two verified copies somewhere else. Formatting before that means a single drive failure loses the shoot, and card formatting is the step people rush because it is the one standing between them and the next job.
Doing it on a Mac
GetFerret offloads cards with verification built in. It copies, reads back, compares checksums and tells you plainly whether every file matched. Cards formatted exFAT, which is nearly all of them, are handled the same as any other volume.
Because it catalogs as it goes, the footage stays searchable after the drive is unplugged and shelved. Six months later you can find out which archive disk holds a particular clip without connecting anything, which is the other half of the problem with working off a stack of drives.
If you would rather not use an app for it, the principle matters more than the tool. Copy, verify, keep two copies, then format. Any workflow that does those four things in that order is a good one.
A note on card health
Cards wear out, and they tend to fail at the end of their life rather than gradually. Retire a card after a couple of years of steady use, or sooner if a verification ever fails on it. A card that has produced one mismatch is telling you something, and replacing it costs far less than the shoot it takes with it.
Search every drive you own, plugged in or not
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