The firefighter/emergency medical technician in a large urban fire/ems department, would not check information on a USB device (although they would bring it along and pass it onto the hospital in case they want it). While they do have computers in their ambulances, the hassle of plugging in the USB and opening it would be time better spent treating the patient.
If the patient is conscious and able to talk to them, then they would much rather ask them the medical questions for which they need answers in order to build rapport with the patient and be able to ask the follow-up questions.
If the patient is not conscious, then there are medical interventions that they need to be performing rather than messing around with a USB device. The only information they really need about the patient if he/she were found unconscious or has allergies, what medications he/she is currently taking, and any pertinent medical history.
Medical ID bracelets or medical alert bracelets or cards are much more effective at conveying this type of information. However, if in the future USB devices become more common and the format/way to access the medical information becomes more user friendly and standardized, I can see USB devices becoming a viable option for conveying medical information to emergency responders.