Lee,
I contacted Magnolia Cemetery today, but they don't have a record of Robert Hackett being buried there. Since he died in 1863 at Summerville and was from SC, maybe his family took his body back home for burial. Another one of his brothers, John W. (also in the 22nd) was killed at the Crater. He is buried in Petersburg along with many of the other men from this regiment who were killed during that terrible explosion.
Ed