S.M.A.R.T. stands for Self-Monitoring, Analysis, and Reporting Technology.  It is a function of all modern hard drives.  As the name suggests it is a function of the drive that self monitors the drive for stasticts ranging from how many hours it has been on to how many various errors the drive has had in various areas.  Using S.M.A.R.T. monitoring programs you can see the data on the drive.  It is OS independent and all of the data is stored on the drive itself.  Not all chipsets (the main chip of a device) in external enclosures support S.M.A.R.T. requests and as a result any external drives attaching via USB/Firewire may not have S.M.A.R.T. accessable.