questionsumm ... what the heck does this device do?

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It unlocks locked SD cards.

You can use it as a 2.0 compliant USB device (Plug-n-Play) or with the 9V attachment. When you insert the card, the light on the device will start flashing, when finished it will be unlocked.

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I thought that SD cards were locked by the little mechanical switch on them? Is there some kind of a software lock or something?

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Good rule of thumb: If you don't know what it does, then you probably don't need one.