I have an older Asus Essentio CM5570-AP003 with a bad Seagate ST3750528AS SATA II HD and would like to know if I could replace it with an SATA III (WD Caviar Black WD5003AZEX) HD? Can you upgrade this unit to SATA III from II? I called ASUSTek and their support could not answer the question. The thread below touched on this but I am still unclear. My bad Seagate (second one to go bad in 2-years! makes me want to go with a nice and fast WD, more reliable I understand).
The WD is 68 MBps (6 Gbps) Rotational Speed: 7200 rpm Buffer: 64MB and the Seagate is 3 Gbps 7200 32MB.
SATA is not upgradable, but you can get a SATA III PCI card that gives you SATA III connectivity, but to be honest, SATA III with just a hard drive is not really that needed. It will run normally on SATA II.
Seagate and WD drives are equally reliable when you average out the numbers. You just had some bad luck.