Mac and Windows report space differently. On Mac, space is calculated using decimal/Base 10, where 1 GB=1,000,000,000B (so 480,000,000,000/1,000,000,000=480GB). In Windows, space is calculated using binary/Base 2, where 1GB=1,073,741,824B (so 480,000,000,000/1,073,741,824=447GB). An useful analogy would be considering distance in miles vs. kilometers; the distance traveled is the same, but the number used to represent that distance is different.