The ZBook 17 has an SSD storage medium which is better than most notebook PCs that have been on the market for around the same time as it. Most similar spec laptops that have been available 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 many processor core portable PCs that have been available for around the same time as the ZBook 17.
An HDD storage medium is the worst storage medium amongst similar types of laptop.
We think you might also be interested in some of the other notebook PCs that have an SSD storage media currently available including because of it's better review score, the Asus U500VZ.