I have four WD 1002fbys 1TB drives in a RAID 10 array using Intel Matrix manager on an ASUS P7P55d Pro board and an Intel I5 unclocked. Win7 64 with SP1.
What started as a really fast system has slowly degraded. I have done all the operating system upgrades, regularly clean up my registry, am very well protected and clean of viruses. Disk defrag and file optomization done regularly.
Using HD Tune pro shows, for example, a random access time of 15-31ms and low disk read and write rates (varying with file sizes). Only 100gb typically on disk (out of 2tb).
When first fired up i got 260MBps now lucky to get 60MBps. All disks seem in good shape and if I do get a crash rebuild works smoothly and quite fast (8-10 hours with reasonable performance).