This happens to me about once or twice a week. I called sprint to document this. By the end of the call, the card found itself. I tried removing the card to gently clean it. That did not help. It is obviously a software issue. I had the error show up on the notification bar. Then I pulled the card out. When I pulled the card out, a new error flashed up. So that means that the system did actually see the card or who would it have known that I removed it. I then plugged it back in and the error went away.
I am on Sprint. THis is my only complaint about this device. The error usually last about an hour or so and goes away on its own.