WD20EURS - SATA II / 3Gb/s drive, 64MB cache, designed for surveillance use, read/write at 130MB/s
WD20EARS - SATA II / 3Gb/s drive, 64MB cache, designed for desktop use
WD20EZRX - SATA III / 6Gb/s drive, 64MB cache, designed for desktop use, has higher read/write at 145MB/s
http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-701250.pdf
http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-771438.pdf