The problem: a card that looks the same before and after the copy
You copy a card, eject it, put it in your bag. Later, standing at the camera, you hold two or three identical cards and have to decide which ones are safe to format. Nothing on the card tells you. Cameras format whatever you point them at, and if you guess wrong, the only copy of a shoot is gone.
The usual fixes are all manual: a strip of tape, a rubber band on the case, a "full" pouch and an "empty" pouch, a note on your phone. They work until the day someone forgets, which is exactly the day it matters.
The trick: make the card ask to be formatted
A small family of tools, ParaShoot being the best known, takes a different approach: instead
of you remembering which cards are done, the card itself says so. After you mark
it, the camera no longer recognises the card as a normal, formatted card. When you insert
it, the camera shows its familiar prompt: Card error, Format card,
Card needs to be formatted, depending on the brand. A card that opens normally
is one you still need to check. A card that asks to be formatted is one you can format.
Nothing is deleted when a card is marked. The photos and videos are still on it, byte for byte. Only a small piece of the card's own bookkeeping is changed, and it can be changed back.
How it actually works
Every card starts with a small area that describes what is on it: how it is divided, what file system it uses, and where to find the files. The first sector carries a short signature, two bytes that say "this is a valid card." Operating systems and cameras look at that area first; if it does not look right, they refuse to read the card and offer to format it instead.
Marking a card flips the bits of that opening stretch. ShotGrab inverts the first 2 MiB of
the card. Your files are far away, later on the card, and are not touched. Inverting is its
own undo: flip the same bytes again and the original comes back exactly. A normal card
carries the signature 55 AA; a marked card carries AA 55. That
difference is all the camera needs to say "unformatted."
Why it can be undone, and when it can't
Because inversion is symmetric, restoring a marked card is the same operation run a second time. ShotGrab also keeps a backup of those first 2 MiB on your Mac before it writes anything, re-reads what it wrote to confirm it, and puts the original bytes back if anything doesn't match.
- Before the camera formats it: you can restore the card in ShotGrab and read every file as if nothing happened.
- After the camera formats it: that is a normal format. The card is empty and ready, and the old files are gone, which is the whole point. You were meant to have them safely copied by then.
The safety check that makes it worth trusting
A mark is only useful if it means "this card has been copied", not "someone clicked a button." That is the part ShotGrab adds to the idea. It will only mark a card when every file on it has a verified copy in your archive, and that copy is still there when you confirm. If a single clip is missing or changed, it refuses, and tells you which ones.
If your copy lives on one disk only, ShotGrab tells you, and suggests a second backup before you format. It doesn't block you; the decision is yours. But a card that was marked only after a checked copy is a card you can format without a second thought.
What marking doesn't do. It doesn't protect a card from being formatted by mistake in a computer, and it doesn't replace a backup. It tells you what the camera will say when you insert the card. The copy that makes formatting safe still has to exist.
Why it asks for your password
Writing to the first sectors of a card means writing to the raw disk, which macOS reserves for administrators. ShotGrab asks for your password or Touch ID each time, once for any number of cards, and nothing stays installed with extra privileges between marks. The helper that does the writing is signed with the same identity as the app, and re-checks everything itself before it touches a disk: that it is a whole external disk, of the size the app said, with a single real partition.
How to do it in ShotGrab
- Open the Disks sidebar (button at the top left, next to the title) to see the cards that are connected.
- Right-click a card, or select several with
CmdorShift, and choose Mark for formatting. - Confirm with your password or Touch ID. Pull the card out. It now shows up as "to format" in the sidebar until the camera formats it or you choose Restore card.
A routine that needs no memory. Copy the card, let ShotGrab verify and eject it, mark it, and drop it in the bag. At the next shoot, every card in the bag that opens normally still has work to do. Every card that asks to be formatted is ready.