The HP ZBook 15 has an SSD storage medium which is better than most laptops of this age and spec. Most many processor core portable PCs that have been available for a similar time 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 in fact the best storage medium amongst all laptops of a comparable spec and age to the ZBook 15.
Amongst comparable types of laptop the worst storage medium is HDD.
There are a number of other portable PCs that have an SSD storage media currently available which you might also want to look at including the ZBook 17[pl_2036831_price], or the ZBook 17[pl_2036831_price] both from HP which has better reviews; or the HP EliteBook 745[plb_2446437_price], or the DELL XPS 15 9530[plb_2110380_price] which is cheaper.
If the storage medium of this portable PC is not good enough for you then you might want to consider the HP ZBook 15[plb_2036830_price] (SSD).