Statistics about hard drives from Google’s data centers were published in USENIX, showing how different variables affect the failure rate of hard drives. AFR is “Annual Failure Rate”.
The older drives may be failing more simply because they are a less advanced batch.
Low, medium, and high specify the usage.
Surprisingly, 35-45 degrees C is the sweet spot for hard drives. Colder temperatures actually cause them to fail more.
SMART will catch about half of hard disk failures before the happen, but many drives will fail without any warning.
Pinheiro, E., Weber, W., & Barroso, L. (2007). Failure Trends in a Large Disk Drive Population. Proceedings from FAST ’07: USENIX Conference on File and Storage Technologies, 5