TechARP posted a review on the Western Digital Scorpio Blue 640 GB Hard Disk Drive

There are two fundamental differences between Western Digital's Scorpio Black and Scorpio Blue drive families - spindle speed and storage capacity. The Scorpio Black drives spin at 7,200 RPM and are currently limited to a maximum capacity of 320 GB. The Scorpio Blue drives, on the other hand, spin at a more sedate 5,400 RPM but have a maximum capacity of 1TB.

While Western Digital offers Scorpio Blue drives in capacities of 1 TB and 750 GB, these models are 31.5% thicker than standard 2.5" drives and may not fit most notebook storage bays. They also spin at a slower spindle speed of 5,200 RPM. The model with the largest storage capacity, that also conforms to the standard 2.5" form factor, is the 640 GB WD Scorpio Blue hard disk drive, and this is the drive we will be looking into today.

Western Digital Scorpio Blue 640 GB Hard Disk Drive Review