The HP Z640 has an HDD storage medium which is faster than the Flash storage medium found on most similar spec personal computers that have been around for around the same time as 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.
Alternative HDD PCs currently available which you might also want to consider include the Z620[pl_2190153_price], or the Z440[pl_2665760_price] both from HP which is cheaper.
If you want a PC with a faster storage speed then you might also want to consider the HP Z440[plb_2466558_price] (SSD), or the Lenovo Thinkstation P500[plb_2535509_price] (SSD).