How do you attach a simple LED activity light that displays an
indication of data flow on a  Serial-ATA (SATA) data line?
There are 4 data lines (2 duplex lines) on a SATA data line.  I am
interested in monitoring the data flow from the hard drive back to the
controller.  I do not want to corrupt the data line while attaching an
LED device.
A good answer would be provided in a schematic or wire diagram form
along with a succinct engineering explanation.

Check www.xbox-scene.com and look at there mod tutorials.  I added an
hdd activity light on my xbox and it is basically the same thing.