Well In my case the data actually is in the laptop and encrypted, but the SD card only disables logging in on the PC, even if they crack my password.
That's at least what I imagined, his setup might be completely different (a keyboard combination for logging out might also work better).
if the SD card is not mounted when the user logs in the system is automatically shutdown too.
Basically the SD card only fights against using the computer. You of course could remove the hard drive and mount it in another computer, but that is why the whole drive is encrypted.