The data ______ been transcribed.
Most Americans would say has. I spent 20 years in data processing and people would look at me strangely if I said "the data have". I know data is supposed to be plural, but popular usage treats it as a collective noun, e.g. herd. The herd of cattle has.....
In science and many other technical documents, the plural is still used where appropriate.
data is the plural of datum. Period.
It depends on the the context (the author's usage). Generally, in scientific text, datum and data are used correctly. However, nowadays, especially in humanities text, the word data is commonly used as a singular word!
Datum is singular, data is plural. However, data is now commonly used as either in English language, another bastardisation of proper language