Grammatically, he should be 'Datum'.
Reposted from
Holodeck Four
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https://xkcd.com/1429/
https://xkcd.com/2305/
"Mr. Data were on the bridge"
He's "Mr. Data" the way Sheldon Cooper's brother is "Dr. Tire": it's a colorful moniker describing a prominent trait, like being made of data or selling tires.
Also, it'd be "Mr. Data are on the bridge."