Here’s a little known fact about SD Cards. The little “lock” switch on the side of the card? It doesn’t lock the card.


This is a photo of a pretty typical SD card. On the left you can see the “lock” switch. You’d think that sliding this switch downwards, into the locked position, would write-protect the card. You’d be wrong.

I first stumbled across this fact on vacation, where at an internet cafe I ended up with a virus on a “locked” SD card.

The lock switch does nothing to the internals of the card. There is no switch or any electrical mechanism in the card. That switch is nothing more than a movable piece of plastic. The card itself has no idea whether the switch is in the locked, or unlocked, position.

In your card reader is a mechanism which senses the position of that lock switch, and reports its position back to the PC. The PC, ie the operating system or file manager in the PC, then has the option, if it so chooses, of not writing to the card if it knows the lock switch is in the locked position. However, it can just as easily ignore it. Electrically, there is nothing preventing it from writing to the card. This is what a virus can easily do. If there is a problem with your OS or file manager program and it ignores the switch position, it too will happily write to a “locked” SD card.

So beware. Don’t assume your card is safe simply because the switch says it’s locked. The card can still be written to, re-formatted, the card contents erased, etc. Accidents most certainly can happen, even though the switch is “locked”. It’s not locked, it’s never truly locked, so treat your card, and its contents, with all appropriate due care & respect. I’ve no idea why the people who designed the SD card implemented a lock switch that doesn’t lock, but they did, and we’d be wise to remember that little fact.