1. An image-forming apparatus to connect to an external memory, the image-forming apparatus comprising: a memory to store a job; a network interface to communicate with the external memory; and a control unit to compare a data size of the job to an available memory size of the external memory with reference to a look-up table and to determine whether the job can be stored in the external memory based on the comparison, wherein the look-up table includes a plurality of data sizes corresponding to a plurality jobs having various combinations of resolution, color mode, and document kind values.