Something that is rarely mentioned is drive reliability. The spec sheets for the most drives I've seen, including the WD black drives, show an unrecoverable read error rate of up to 1 per every 10^14 bits read. Since a 1TB drive contains about 10^13 bits, that means as much as a 10% chance of not being able to successfully read all of the blocks on the drive. For a 2TB drive it's a 20% chance.
One of the reasons I like the WD Green drives is because they have an unrecoverable read error rate that's 10 times better - up to 1 error per every 10^15 bits read. That translates to only around a 1% chance of an unrecoverable error reading every block of a 1TB drive, or 2% for a 2TB drive.