Ah, okay. Thank you.
Well, it was a physical problem with the card reader. Popped it open and corrected the problem. The silver cover had loosened and the SD card wasn't making contact with the pins since the cover wasn't there to "push" it onto the pins.
I'm not a Linux guy and it was...
Yes, I've tested it with two cards.
This doesn't seem like a problem with the SD reader itself - it seems like a problem with where the file system is telling the GP to read/put files. . .