I know that in "proper" English usage, it is "data are", but programmers always use "data is".
I think it is because programmers are logical.
If it were "data are" then the following should make sense:
Jim: My data are insufficient.
Ed: How many data do you have?
Jim: Let's see, I had 15 data yesterday and got a new datum today, so I now have 16 data.
Ed: Well that's not very many data, OK.
But nobody ever says "I don't have many data".
Everybody says "I don't have much data"
And nobody counts it.
Data is not countable, like employees or dollars. It is abstract, like information or water.
Nobody says "The water are cold today". Why say "The data are not all being read?"
It is rediculous to say that "data" is the plural of "datum". That usage is arcane and obsolete. Just as "information" is not the plural of anything, so "data" is not the plural of anything. It is just stuff.
Whoever uses "data are" is (derogatory description deleted)
I agree with Mac that "data" should be considered as a collective noun (or a "mass noun"), and that "datum" should be retired.
(I also consider "garbage" a collective noun!)
Note: A collective noun can be considered either as a group of objects or a single object. The exact definition of a genuine collective noun is debatable. The following definition is taken from The American Heritage Book of English Usage. According to that reference (see the last paragraph), "data" might be considered a "mass noun."
"Some nouns, like committee, clergy, enemy, group, family, and team, refer to a group but are singular in form. These nouns are called collective nouns. In American usage, a collective noun takes a singular verb when it refers to the collection considered as a whole, as in The family was united on this question or The enemy is suing for peace. It takes a plural verb when it refers to the members of the group considered as individuals, as in My family are always fighting among themselves or The enemy were showing up in groups of three or four to turn in their weapons. In British usage, collective nouns are more often treated as plurals: The government have not announced a new policy. The team are playing in the test matches next week.
Be careful not to treat a collective noun as both singular and plural in the same construction. Thus you should say The family is determined to press its (not their) claim.
Collective nouns always refer to living creatures. Similar inanimate nouns, such as furniture and luggage, differ in that they cannot be counted individually. That is why you cannot buy a furniture or a luggage. These nouns are usually called mass nouns or noncount nouns. They always take a singular verb: The bedroom furniture was on sale."
As far as I can see, data comes from the Latin verb for giving (do/dare/dedi/datum) and I believe that the word for "thing/things/stuff" is omitted.
So data refers to something which is "given", but my textbook suggests that the plural would be datae...
Datum is actually the neuter form, while data is feminine. (the "thing" word, RES, is feminine)
Interesting debate... my Latin text book does affirm that the data in use today comes from Latin but gives no more information.