Specs for Western Digital Caviar Black (high performance 7200RPM drives): WD Black
2TB, read/write power 10.7W. 1TB, 6.8W. (I didn't expect that large a difference. The 4TB unit also only needs 10.4W.)
I don't know what a "2A" power supply means, exactly. (2A at 12V?) Is the USB adapter rated to be able to use more power than that?