It is an 8TB HDD but it has to do with how the drive is read. HDD have been that way for years when it comes to showing the amount free. I got this from Seagates website to explain the discrepancy:
"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 one terabyte (TB) is equal to 1,000,000,000,000 bytes.
Programs such as FDISK, system BIOS, Windows, and older versions of macOS use the binary (base 2) numbering system. In the binary numbering system, one megabyte is equal to 1,048,576 bytes, one gigabyte is equal to 1,073,741,824 bytes, and one terabyte is equal to 1,099,511,627,776 bytes."
My guess is since Xbox One is based on Windows 10 it's still using the base 2 numbering system. So your drive is fine. I got one too recently to use on my Xbox One X and it has been great so far!