WD Red filling up twice as fast

Aug 30, 2018
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I recently purchased a 4tb WD Red for NAS video storage. I transferred the contents of a folder totalling 1.9 tb, and when it finished it looks like the drive is almost full! Only a few hundred gigs left.
I'm not sure I understand why it is filling up twice as fast.
Is it in raid with itself due to my mistakes? Is that just how Red hdd's work? If I had known that a 4tb hard drive is really 2tb, I would have gotten a higher capacity.
Thanks in advance!