Where is S.M.A.R.T. data stored on a HDD?

As mentioned in this informative blog post, and as confirmation to @djg's answer, some manufacturers store some information in the service area or negative track area of a drive, while others use memory on the PCB, as mentioned by @Dracs.

The hard drive system tracks start at track -1 and can go to track -6, depending on model. These system tracks are only accessible via the hard drive control logic and not visible to operating systems. The system tracks contain SMART data as well as other data such as serial number, etc.