I just bought a 500GB Passport HDD and after connecting it, the system clearly shows it has 465GB free. I do know that 1024mb = 1GB but that still doesn’t account for 20GB of missing space.
The space is not missing, is just a consequence of the format system. Normally the external hard drives are formatted using FAT32 system and this produce some lose of space. If you want recover some space you can format your drive using NTFS system.