The ZBook 15 has a better storage medium than most comparable spec notebook PCs that have been available for around the same time as it. This notebook PC has an SSD storage medium, whereas most portable PCs of a similar age and type to 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.
This portable PC's storage medium (SSD) is actually the best storage medium amongst all portable PCs of this type and age.
The worst storage medium across comparable types of portable PC is HDD.
If the storage medium of this notebook PC is too bad for you then you might want to look at the DELL Latitude 12 E7250 (SSD).