Desktop Drives are cheap(er), but are not suited for use in a RAID array with a proper RAID controller. Why is that? Were the drive manufacturers pulling wool over our eyes so that they could sell more drives?
Western Digital put out a KB article to explain how desktop drives were different from Enterprise drives, and why you should not put desktop drives into a RAID array. http://wdc.custhelp.com/app/answers/detail/a_id/1397/~/difference-between-desktop-edition-and-raid-(enterprise)-edition-drives
The bottom line is that WD drives have a special feature for automatic recovery within the drive electronics which will interfere with the automatic recovery code in the RAID controller. The end result is a hard drive failing in the array, when it actually hasn’t. It is worth repeating the Critical Caution note here:
Critical: WD Caviar Black, Caviar Green, and Caviar Blue hard drives are not recommended for and are not warranted for use in RAID environments utilizing Enterprise HBAs and/or expanders and in multi-bay chassis, as they are not designed for, nor tested in, these specific types of RAID applications. For all Business Critical RAID applications, please consider WD’s Enterprise Hard Drives that are specifically designed with RAID-specific, time-limited error recovery (TLER), are tested extensively in 24×7 RAID applications, and include features like enhanced RAFF technology and thermal extended burn-in testing.