David McCallum, known for his role as Dr. Donald "Ducky" Mallard on "NCIS," has passed away at the age of 90. Despite his iconic performances in shows like "The Man From U.N.C.L.E." and "Colditz," McCallum was best known for his portrayal of Ducky on "NCIS" for 20 years. However, fans will not see any new scenes featuring McCallum in "NCIS" Season 21, as no episodes have been filmed due to strikes. McCallum's final appearance as Ducky was in the Season 20 finale earlier this year.