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A write-protect notch is a cutout or opening on some removable media that a drive uses to tell whether writing should be allowed. The term is most often used for floppy disks. Full-size SD cards have a related sliding write-protect switch, but it works as a signal to the host device rather than as an internal lock in the card.
What does “write-protect notch” mean?
It is a physical feature that helps prevent data from being written to removable media. The notch, opening, cover, or tab changes what a drive or host detects; the direction that means “protected” depends on the media format.
Write protection is the broader term. The NIST glossary defines it as “Hardware or software methods of preventing data from being written to a disk or other medium.” A notch is one physical way to signal a protected state, not a synonym for every kind of write protection.
How floppy-disk notches and tabs work
On floppy disks, the drive reads the disk’s physical feature and inhibits writing when it detects the protected state. Apple’s IIgs manual calls the feature a “write-enable notch”: a small square cutout on a 5.25-inch disk or a small square hole on a 3.5-inch disk. The manual describes using a tab to cover or uncover the opening; for 3.5-inch disks, it describes a sliding plastic tab. Apple IIgs Owner’s Manual.
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The protected position is not universal across every floppy format or mechanism. Brother’s manual, for example, says to slide the notch to open the slot to protect the disk. It states, “A write-protected disk is read-only.” Brother manual.
How an SD-card write-protect switch differs
A full-size SD card has a sliding mechanical switch on its edge. The SD Association’s format overview lists a write-protect switch for full-size SD cards, but not for microSD cards.
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The switch is a read-only request interpreted by the device using the card. The cited SD-card reference explains that the card itself does not detect the switch position; the host is responsible for honoring it. So it should not be treated as a tamper-proof lock built into the card’s circuitry. The same reference notes that miniSD and microSD cards can be used in full-size adapters with a switch.
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| Mechanism | Physical feature | Who reads it | What it means |
|---|---|---|---|
| Floppy disk | Notch or opening, sometimes covered by a tab; protected position varies by format | Floppy drive | The drive inhibits writing when it detects the disk’s protected state. |
| Full-size SD card | Sliding switch | Host device | A read-only request for the host to honor; the card does not itself detect the switch position. |
What a write-protect notch does not guarantee
- It does not define one universal switch direction: consult the instructions for the specific disk or device.
- It is not the same as software write protection. NIST’s definition includes both hardware and software methods.
- An SD-card switch is not an internal, tamper-proof lock; the host device is responsible for responding to it.
- A microSD card does not have the full-size SD card’s built-in sliding switch. An adapter may provide one.
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