A printing apparatus which is capable of carrying out proxy printing by a simple operation of attaching a removable storage medium, while preventing an erroneous operation without complicating an operation for selecting a function from various functions for use with the removable storage medium. The printing apparatus prints out print data stored in a storage device. When the printing apparatus cannot print out the print data stored in the storage device, print data stored in the storage unit and yet to be subjected to print processing is transferred into the external memory in response to attachment of the external memory to the printing apparatus. When the printing apparatus can print out the print data stored in the storage device, a job is executed using the external memory in response to attachment of the external memory to the printing apparatus.