The DDN 600GB SAS 15K RPM 3.5" drives are being replaced with 600GB SAS 10K RPM 2.5" drives fitted in 3.5" caddies.
The 3.5" 600GB drives are no longer available.
The 2.5" drives are functionally equivalent, and overall the same performance. The technology behind this is higher areal density of the 2.5" drives, they are higher than that of the 3.5", this therefore provides the same or better bit rate from the head / media offsetting the slower RPM rate.
This applies to drives for SS6000, SS7000 and SS8000 DDN drive enclosures.
The DDN part numbers for the SS6000 enclosures and additional information are:
6K-600G6SC2QN = 2.5" 600GB 10K = HUC109060CSS600
SS360-0600QZ = 3.5" 600GB 15K = ST3600057SS = HUS156060VLS60