A write-protect notch is a cutout or opening on some removable media that a drive reads to decide whether writing should be allowed. The exact meaning depends on the format: floppy disks use notches, covers, or tabs that a drive detects, while a full-size SD card’s sliding switch signals a read-only preference to the host device.
What “write protection” means
Write protection is the broader concept; a notch is just one way to signal it. The NIST glossary defines write protection as “Hardware or software methods of preventing data from being written to a disk or other medium.” That definition covers more than physical cutouts and does not mean every method offers the same level of security. NIST glossary
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How floppy-disk notches and tabs work
On floppy disks, a drive detects the disk’s physical opening or covering and inhibits writing when it recognizes the disk as protected. Apple’s manual calls the relevant 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. Covers or tabs change whether the opening is exposed, but their direction and construction vary by format. Apple IIgs manual
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There is no universal rule that an open notch always means “locked.” Brother’s manual, for example, says to slide the notch open to protect the disk. Apple’s instructions describe covering the write-enable notch on a 5.25-inch disk with a tab, and a sliding plastic tab that covers or uncovers an opening on a 3.5-inch disk. Follow the markings or instructions for the particular disk and drive. Brother describes the result plainly: “A write-protected disk is read-only.” Brother manual
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How an SD-card lock switch differs
A full-size SD card has a sliding mechanical write-protect switch on its edge. The SD Association lists this switch in its overview of SD card features. The switch is not an internal circuit that blocks writes: the card itself does not sense its position. Instead, the host device reads the position and is responsible for honoring the read-only request. This makes the switch a useful handling safeguard, not a tamper-proof or card-enforced lock. SD Association overview · SD card reference
Which cards have the physical switch?
The distinction matters when choosing an adapter or diagnosing a card. The SD Association overview lists a write-protect switch for full-size SD cards, not microSD cards. MiniSD and microSD cards may be used in full-size adapters that have a switch; in that case, the host interprets the adapter’s switch position. SD Association overview · SD card reference
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At a glance: notch versus SD switch
| Mechanism | Physical feature | What reads it | What protection means |
|---|---|---|---|
| Floppy disk | Notch or opening, sometimes covered by tape or a movable tab | Floppy drive | The drive inhibits writing when it detects the disk’s protected state, as described in the cited manuals. |
| Full-size SD card | Sliding tab | Host device | A read-only signal or request; the SD card itself does not detect the switch position, according to the cited secondary reference. |
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