Hi Please find the HP tested HDD on the z240 Tower and SFF unit as given below, the highest specs,
HP Z240 Tower Workstation :
4TB SATA 7200 rpm 6Gb/s 3.5" HDD, Part # K4T76AA
HP Z240 SFF Workstation
3TB SATA 7200 rpm 6Gb/s 3.5" HDD, Part # QF298AA
Please note "WD 6TB 7200 RPM SATA III 128MB Cache Enterprise HD (WD 6001FSYZ)" is not one of the hard drive tested on this unit z240.