Joe probably what you need to do is look under all the alternate spellings of Buchanan you can find in other infantry AND cavalry units. I found Robert T. as Buchanan, and Buchannon in the book Tennesseans in the Civil War, part 2. Both of these are in Co. B, 50th Infantry Regiment and are most likely the same person. This company was organized at Cumberland City according to Tennesseans in the Civil War. The fact that he dropped off the radar may or may not mean he left his unit for good. My gg grandfather Newton left the 32nd Tn. in May of 1863 and I haven't found anything positive for him after that myself either.
Here is the site for the 50th Regiment that might be helpful. The 50th seems to have consolidated with other units part of the time.

http://www.tngenweb.org/civilwar/csainf/csa50.html