It might be quicker to just encrypt the entire partition or create an encrypted partition. The data will be decrypted and re-encrypted as it moves between the NAS and the HD, but performance will likely be better. Alternatively, if you have a formatted partition in Windows, and you right-click on it and click properties, you'll get a "free space" reading that shows you the number of bytes. From there, you can do any math that is necessary to get the number you need. For instance, part of the reason you see a lower number in Windows is because it technically uses TiB/GiB/MiB/kiB vs TB/GB/MB/kB that HD manufacturers use. While I don't remember what TrueCrypt requires, if it has 7,997,229,105,152 bytes free, you could try dividing by 1,000,000 to get MB, and if that was too big, you could try dividing by 1,048,576 (1024*1024) to get MiB (even though Windows calls it MB).