External device and operating method applied thereto

An external device is used with a backend host operated with an operating system. The external device has a program and receives incoming data. The external device is operated with the backend host according to an operating method including the following steps. Firstly, the external device is connected to the backend host, and the external device is recognized by the operating system. Then, the external device is reconnected to the backend host to have the operating system recognize the external device as a memory device when the external device is unrecognizable by the operating system, so that the program is accessible and executable by the operating system to control the external device, or the external device converts the incoming data into at least one data file complying with the access format of a certain program in the operating system.