The HP ZBook 17 G2 has an SSD storage medium which is better than most many processor core laptops that have been around for a comparable time to it. Most similar spec notebook PCs that have been on the market for around the same time as it have an HDD storage medium. Solid State Drives (SSD) store data in semiconductors on a circuit board rather than using magnetic or optical media. Solid State Drives have no moving parts which makes them very fast and reliable.
An SSD storage medium is in fact the best storage medium amongst all laptops of a comparable age and type to the ZBook 17 G2.
The worst storage medium across comparable types of laptop is HDD.