[jacobolus]

If you have a 2%/year failure rate per drive, then that leaves you with a nearly 12%/year chance that at least 1 of 6 drives will fail each year. Or a 31% chance that at least 1 of 6 drives will fail within 3 years. Or a 52% chance that at least 1 of 6 drives will fail within 6 years.

[sangnoir]

The question then is, would it be cheaper to replace that one drive or get the more expensive disks with lower chances of failing?

[Arn_Thor]

Isn't there a cost to time and convenience too? Buying a new disk takes time, as does rebuilding the RAID. And during that time you are vulnerable to another drive failure which could be disastrous if you only have one drive redundancy, especially during the very intensive rebuilding process.