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According to Wikipedia, the SD card reader recognizes if the little tab on the left of the SD card is in the "lock" position or not. Thus, a SD card reader can still write on the SD card even if the write protection is on by ignoring it.

Is it true that the write protection is only a voluntary feature by the card reader or is it a "switch" that securely turns write access off?

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Wikipedia is correct: on SD cards, you have to trust the host system (whatever the card is plugged in to) to honor the physical write protect switch. Here is the relevant text from the publicly available specification documents. Emphasis is mine.

SD Specifications Part 1

Physical Layer

Simplified Specification

Version 4.10 January 22, 2013

4.3.6 Write Protect Management

Three write protect methods are supported in the SD Memory Card as follows:

  • Mechanical write protect switch (Host responsibility only)
  • Card internal write protect (Card's responsibility)
  • Password protection card lock operation.

Mechanical Write Protect Switch

A mechanical sliding tablet on the side of the card (refer to the Part 1 Standard Size SD Card Mechanical Addendum) will be used by the user to indicate that a given card is write protected or not. If the sliding tablet is positioned in such a way that the window is open it means that the card is write protected. If the window is close the card is not write-protected. A proper, matched, switch on the socket side will indicate to the host that the card is write-protected or not. It is the responsibility of the host to protect the card. The position of the write protect switch is unknown to the internal circuitry of the card.

  • It appears that there is also some hard-wired protection available ("Card internal write protect"), but this is only described in the confidential full specification and I think it is not linked to the write protect switch. – Andalur Mar 14 '14 at 11:06

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