Why a 14 TB external drive appears to be 12.7 TB in size – the short explanation!
The marketing TB of a drive is based on multiples of 1000, so 14 TB is 14 x 1000 x 1000 x 1000 x 1000 = 14,000,000,000,000 bytes.
Computers use the binary number system, so the computer’s view is based on 2^10 = 1024 instead, so 14 TiB (“tebibytes” or binary terabytes) = 1024 x 1024 x 1024 x 1024 = 15,393,162,788,864 bytes.
Computers show TiB, not TB, but this “iB” convention is relatively new and adoption of the new unit names has been slow (I use it in programs I write but I don’t work on Windows Explorer code). You are not being ripped off when you get a 14 TB drive and see a lower capacity in the drive properties; this is simply an issue of differing units. The “iB” units were created to disambiguate and solve this problem, but until they see wider adoption, these sorts of misunderstandings will occur.
Personally, I think drive manufacturers should have been specifying binary multipliers all along, but as you have seen, the decimal multipliers let them sell a “14 TB” drive which is 12.7 TiB, so the bigger number looks better and wins in the marketing department. This will likely never change, so get used to it.