A diagnostic tool includes a security system which prevents the unauthorized downloading of updated software. In one embodiment, a first external storage device includes a first security signature and is electrically coupled to the diagnostic tool. A second external storage device includes software and is also electrically coupled to the diagnostic tool. When a second security signature located within the diagnostic tool is the same as the first security signature included within the first external storage device, the diagnostic tool downloads the software into an internal storage device located within the diagnostic tool. In another embodiment, an external storage device includes software and is electrically coupled to the diagnostic tool. Upon initial use, a first security signature which is the same as a second security signature located within the diagnostic tool is written by the diagnostic tool to the external storage device. Thereafter, if a user attempts to utilize the external storage device with a different diagnostic tool, that diagnostic tool will not run the software located on the external storage device.