there are serveral land warrants in Ark and land purchases of a Frederic and Frederick Harwell, any of which might be Frederick Harwell of TN.
So, he might have had land or moved back and forth. Don't know. Need to research the counties further to see if the son's might have went to Ark during the Civil War.
Sarah A. L. Spencer aka Sack married 1st Erasmus Harwell, 2nd Jesse Ross. No marriage record found for Sarah and Erasmus, but they are in the 1860 Census together near Frederick and Nancy, with no children. Sarah is a widow in 1880.