Daniel You are right. the bottleneck is not caused by SATA. I failed to mention that the pool I have created on WS2012r2E uses parity as a redundancy measure. Apparently, the parity is to blame for low write speeds - known issue applicable only to writes. Reads are fine. Thanks for responding
Gents I inherited PowerEdge 1900 and decided to migrate my old WHS2011 to it. I have 10 SATA drives to transfer (various sizes 1TB - 4TB). I have started by moving 3x 2TB drives and connecting them to SATA B,C & D ports. SATA A is occupied by SSD. Now everything went sooth (hardware wise) all drives are recognised, 3x 2TB drives are added to WS2012r2 Essentials Poll. I have started migrating data to release other drives which I am hoping to connect to Perc 5 and other PCIe SATA cards, however what I have noticed is that the write speeds (when moving data over GBit NIC) are 33.5MB/s with occasional peaks of 67.4 MB/s. I think this is due that one of the SATA Ports (B,C or D) is ATA and limited to 33MB/s. Unfortunately I am struggling to find info on this.  Anyone knows the speeds of embedded SATA ports? or how can I improve transfer speeds? As mentioned, I would have quite a few drives to move so every port counts.  Thank You
PowerEdge 1900 & WS2012r2