An image forming apparatus that is capable of accessing a removable storage medium and automatically starting an operation of calibration thereof without waiting for a user to input a job, on an as-needed basis, when a storage medium is mounted therein. A printer engine performs an image forming operation according to data read from the medium, and a controller executes calibration of the printer engine. The controller detects whether or not the medium is inserted, and determines attributes of data stored in the medium. Based on a result of determination, the controller causes the calibration to be selectively started.