The HP Z420 has an HDD storage media which is faster than the Flash storage media found on most many processor cores desktop computers that are a comparable age to it. A Hard Disk Drive (HDD) is the traditional storage method for computers. A HDD stores the information on magnetic disks. HDD drives are cheap, but they are slower than Flash and SSD bases storage systems.
An HDD storage media is actually the worst storage speed amongst all desktop computers of a similar age and spec to the Z420.
Amongst similar types of personal computer the best storage speed is SSD.