An image processing apparatus that is capable of extending a function while guaranteeing performance and quality. The image processing apparatus has a USB host controller. A first USB host interface is installed inside the image processing apparatus. A storage unit stores device information about a USB storage device that can be used as a system storage device of the image processing apparatus. A memory device stores a set of instructions. A processor executes the set of instructions to obtain individual identification information about a USB storage device from the USB storage device in a case where the USB storage device is connected to the first USB host interface, and control to achieve a state where the USB storage device is available as the system storage device of the image processing apparatus in a case where the individual identification information is in the device information.