The ZBook 15 has an SSD storage medium which is better than the HDD storage medium found on most portable PCs of this type and age. 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 that have been available for around the same time as the ZBook 15.
The worst storage medium across similar types of portable PC is HDD.
There are a number of alternative notebook PCs that have an SSD storage media currently available which you might want to consider including the NP905S3G, or the NP905S3G both from Samsung which has better reviews.