The blindeagle external units have an internal memory which contains your encryption key. For every message or e-mail, sent or received, the external device consumes some of its encryption keys. This means that every device has a limited life span.
Therefore, external units with bigger memories will have a longer life span.
Once your external device is empty you will be able to purchase another one and simply use the blindeagle app to get an account transfer on your new device.