From: utzoo!decvax!genradbo!grkermit!chris
Newsgroups: net.nlang
Title: Re: Using data as a singular noun
Article-I.D.: grkermit.301
Posted: Fri Feb 25 15:33:14 1983
Received: Sat Feb 26 03:31:12 1983
References: floyd.1217

The term you want to use instead of "mass noun" is "non-count noun."
The opposite is obviously "count noun."  This is a reasonably well
established concept.  (I learned it in the fifth grade or so.) The term
is used to distinguish between nouns for which the plural form
indicates a discrete number of the item, and those for which the plural
indicates an undistinguished jumble of the item.  Other non-count nouns
are "water", "sugar", and "mail."

I think that "data" and "datums", (considered an acceptable plural form
by most dictionaries) should be treated as non-count nouns, and so your
usage was correct.