When you purchase a 4TB drive, you may only get 3.6TB usable storage. This is because the hard drive manufacturers' market drives in terms of decimal (base 10) capacity. In decimal notation, one megabyte (MB) is equal to 1,000,000 bytes, one gigabyte (GB) is equal to 1,000,000,000 bytes, and so on. But the Windows computer reports in binary, this is the difference. For 4TB, multiply that by 4 to get 3.638 TB of actual available space for Windows.