I have 3 640GB black edition drives. Two WD6402AAEX and one WD6401AALS. One of WD6402AAEX (the new one - bought recently) is slower than the other (and also slower than WD6401AALS).
For example, HD tune read test (same settings, same SATA III port, same system, same firmware):
WD6401AALS - maximum read speed: 116MB/s
WD6402AAEX (the “old” one - SN: WCATR060xxxx) - maximum read speed: 133MB/s
WD6402AAEX (the “new” one - SN: WCATR202xxxx) - maximum read speed: 106MB/s
WD6402AAEX Performance Specifications:
Transfer Rate (Buffer To Disk): 126 MB/s (Max)
Any suggestions? Note: Local vendor tested for me another WD6402AAEX with same poor result.