The main difference between pastrami and corned beef is the flavor and final preparation. Both meats are cured in a nearly identical way, in a brine containing salt, curing salt, and flavorings. (This curing is a remnant of an age before refrigeration when a whole cow would have to be slaughtered but could certainly not all be eaten at one time. The curing helped to keep the meat palatable and safe for longer.) But the similarity stops there. Corned beef gets boiled and served as is, but pastrami gets its coat of spices, a hit of smoke, and then a steam bath to loosen it up. But if you take a core sample from pastrami that is sufficiently far from the smoky, seasoned surface, it will be very hard to tell the difference between it and corned beef.