A method of writing data to a protected region in response to a request from a host includes receiving a first write request including a first host message authentication code and a first random number from the host, verifying the first write request based on a write count, the first random number, and the first host message authentication code, updating the write count based on a result of verifying the first write request, generating a first device message authentication code based on the updated write count and the first random number, and providing the host with a first response including the first device message authentication code and a result of the verifying of the first write request.