What is the difference between the short and long self-tests?

Short tests do a quick scan of the HD, which takes only about 2 minutes. Long tests do a sector by sector scan of the HD, and hence takes about 80 minutes or more depending on the drive.
<- How often does SMART Utility check or poll the drives? Why are all of my external drives showing up as UNSUPPORTED? Why can't SMART Utility scan these? ->