When an external device connects to a connecting port of a host, a current detection circuit of the external device detects if the current provided by the host is less than the working current needed by the external device. If the current provided by the host is insufficient to initialize and operate the external device, a controller of the external device then stop powering the main component of the external device so that the main component will not damage due to constantly start/stop and can be protected. Meanwhile, an indicator of the external device notifies the insufficiency of current provision by vibrating, beeping, or displaying an indicating signal. Illusion of failure of the external device may be effectively avoided.