Buffer
"Data" in a computer context is not the same thing as "data" in some other contexts. Just as it's quite proper to say indexes when you're referring to data structures, you shouldn't refer to computer data as plural. After all, what is a computer datum? is it a bit? A byte? The contents of one field in one record of a database? Has anyone since the early days of computing even referred to a computer datum in English?
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".