The lock switch on an SD card
A tiny slider on the side of a full-size SD card says "lock". You would assume it locks the card.
Category: Keys & screens
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Fault. It only asks politely.
The slider does not physically block writing. It sets a flag that the device reading the card is supposed to respect, so a device that ignores it can still write. The card cannot enforce it alone.
Source: Secure Digital, Wikipedia.