Sure, but who's going to pick up a random USB-to-SD adapter from the parking lot and plug that into a computer? The point of the USB key experiment is that the "key" form factor advertises "there is potentially interesting data here and your only chance to recover it is to plug this entire thing in wholesale".
You're moving your own goalposts, by now restricting this to a storage device that is fitted into an adapter to make it USB. There is no requirement to limit this to USB, however.
They'll pick up the SD/TF card and put it into a card reader that they already have, and end up running something just by opening things out of curiosity to see what's on the card.
One could pull this same trick back in the days of floppy discs. Indeed, it was a standard caution three decades ago to reformat found or (someone else's) used floppy discs. Hell, at the time the truly cautious even reformatted bought-new pre-formatted floppy discs.
This isn't a USB-specific risk. It didn't come into being because of USB, and it doesn't go away when the storage medium becomes SD/TF cards.
> You're moving your own goalposts... This isn't a USB-specific risk
I'm not, because I am talking about a USB-specific risk that has been described repeatedly throughout the thread. In fact, my initial response was to a comment describing that risk:
> A USB can pretend to be just about any type of device to get the appropriate driver installed and loaded. They can then send malformed packets to that driver to trigger some vulnerability and take over the system.
The discussion is not simply about people running malware voluntarily because they have mystery data available to them. It is about the fact that the hardware itself can behave maliciously, causing malware to run without any interaction from the user beyond being plugged in.
The most commonly described mechanism is that the USB device represents itself to the computer as a keyboard rather than as mass storage; then sends data as if the user had typed keyboard shortcuts to open a command prompt, terminal commands etc. Because of common controller hardware on USB keys, it's even possible for a compromised computer to infect other keys plugged into it, causing them to behave in the same way. This is called https://en.wikipedia.org/wiki/BadUSB and the exploit technique has been publicly known for over a decade.
A MicroSD card cannot represent anything other than storage, by design.
