It might be helpful to mention that some of the families in Colleton and Barnwell Districts in the 1850 and 1860 censuses that were listed as "m" or "mu" or "mulatto" were really Native Americans or Indians. Some free blacks of mixed ancestry may have also had this designation, but from years of looking, it is clear that many of these families were Indian.
Why wasn't "I" used? I am not sure, but these decades after the forced Indian removal from much of the South were probably "touchy" for census takers, and even more so for the remaining Indian families themselves.
If you have Indian ancestry from the Cottageville area, look for possible links to the Cusso-Natchez group of the Givhans area. If you have links to the Smoaks and Bamberg area, look for connections to the PeeDee Indians of those areas. Half of the PeeDee tribe was relocated to the Neeses area of Orangeburg County, SC, in the mid-1700s by the English authorities. These people gradually extended their area both across and down the Edisto River.
Among Cottageville - Givhans Indian surnames are most Creels, Davidsons, Tracys, Muckelvaneys, some Blockers, and others.
Among some Bamberg to Smoaks Indian surnames are certain of these families: Proveaux, Kinsey, Neal, Jones, Griffin, Chavis, and others.