USB 3.0 SATA&IDE HDD Converter,20 GB Quantum Fireball Lct 15 HDD

I am selling a USB 3.0 to SATA & IDE Hard Drive Converter plus a 20GB IDE Quantum Fireball 3.5” Hard Drive. I know the Hard Drive says 20GB but in reality it truly on has 19GB, I formatted the drive and that’s what keeps showing up , so I’m not sure what the 1GB of space is being taken up with.