A tiny tab, a decision made by the reader
On a full-size SD card, the sliding tab does not connect to the memory chips or switch off their ability to accept data. Instead, moving it changes what a slot sensor can detect. When the card is inserted, the camera or computer reads that position and can decline requests to write, erase, or format files.
The card’s controller still handles those requests; the tab is a signal to the device using the card. That makes the protection a cooperative rule, not a barrier enforced by the storage itself.
Why the distinction matters
Because the reader makes the decision, behavior can vary. A device that detects the tab may show the card as read-only, while a reader that ignores the position—or has a faulty sensor—may still allow changes. The tab also cannot stop someone from copying files elsewhere or protect the card from every kind of damage or data loss.
It is useful as a quick safeguard against accidental overwriting: slide it before handing a card to a device that might save new photos or reformat it. But it is not a security lock. The surprising part is that the “lock” lives partly outside the card: its effect depends on the camera or computer honoring a small mechanical cue.
