Buffer
I concede that like noöne has ever referred to a singular computer datum, but I think of the word "datum" as simply meaning 'a single piece of information (of any type)'. I am certainly not an expert on programming or computing, so I wouldn't know about their argots. But generally speaking, "data" is always plural and "datum" always singular. And the definition for "buffer" is: "A portion of memory set aside to store data, often before it is sent to an external device . . .", which refers to multiple "data" or "pieces of information" beïng sent to somewhere. If it were just one piece of information beïng sent, then "it is" would be accurate. But since it is referring to multiple (general) data, I think it should be treated as a plural noun. I will conclude by saying that it isn't of supreme importance to me that the definition be fixed; I just personally think it should say "they are".