Pat Magruder has been researching African American descendants in Frederick County, MD, and has found several households, some using the McGruder spelling. She posts her finds on the African American Magruder Descendants Facebook Group, so if you are interested you may want to join that group. Here are her recent posts–
I found a Zeddrick Magruder 73 years old in 1870 and his wife Harriet in Frederick City Maryland African American.
More Mcgruders, Magruders found in Urbana Frederick County , Md. Rueban Mcgruder 1870 census- 25 years old black male. Boarder of Sarah Nailor also black.
Rezin Magruder , white male , age 30 in Frederick City, 1860 census, had an entire African American family named Baton. Charles,his wife Mary and 6 children. Zaddock Mcgruder and wife Priscilla. 1860 census. He was 65 years old. Frederick City. African American. Some of the spelling of Magruder changed to Mcgruder in other census.
Hope this info helps someone.