The file system overhead accounts for some of that. Directories and the file structure overhead (such as bit maps) reside in the Windows File Tables, which aren't "free." They take up space.
Find the window on your system that shows you all the drives including the USB drive letters. Once of those will be your new 1TB drive. Right-click on that to get the device-specific menu. Near the bottom is "Properties." Click that and you will see the free space, used space, and total space on your drive. You will see the exact size down to the byte AND the generically stated size of the drive, in TB. On my system, I have 1,487,008,690,176 bytes = 1.35 TB because they are using terabytes as, not 1000x1000x1000x1000, but 1024x1024x1024x1024.