The ZBook 15 has a better storage medium than most notebook PCs of this spec and age. This HP portable PC has an SSD storage medium, whereas most laptops of a similar spec and age 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.
An SSD storage medium is actually the best storage medium across all laptops of this age and spec.
The worst storage medium across similar types of laptop is HDD.