The ZBook 15 has a better storage medium than most similar spec notebook PCs that came out around the same time as it. This notebook PC has an SSD storage medium, whereas most 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 across all notebook PCs of this spec and age.
The worst storage medium amongst comparable types of portable PC is HDD.
There are a number of other notebook PCs that have an SSD storage media currently available which you might also want to consider including cheaper laptops such as the Toshiba Tecra Z50[plb_2090365_price], the HP EliteBook 850 G2[plb_2949829_price], or the Lenovo ThinkPad X1 Carbon[plb_2831123_price].
If the storage medium of this HP laptop is not good enough for you then you might want to look at the Lenovo X1 Carbon[plb_3046926_price] (SSD).