I share with you my experience. I own a zbook 15 (first GEN). Mine has one 7200RPM 500GB, neither the mSATA or M.2 was populated. I was unable to find a mSATA 256GB so i bougth a M.2 2242 (is the size that notebook supports) 256GB SATA (also exist M.2 PCIe).
I installed the M.2, put the disc...