Connect your Samsung 840 Pro to one of the Intel SATA 6Gb/s ports (SATA3_0 or SATA3_1).
Connect your WD Caviar Blacks to the Intel 3Gb/s ports (SATA2_2 & SATA2_3).
HDDs can be 6Gb/s "compatible", not 6Gb/s "capable".
No HDD can spin fast enough to achieve 6Gb/s speeds.
The 1TB Western Digital Caviar Black has sustained Read/Write speeds of 126MB/s, which is actually SATA 1 (1.5Gb/s) speeds.
The 64MB Buffer in the drive can transfer data at 6Gb/s speeds, but that's it.http://www.wdc.com/wdproducts/library/?id=350&type=8