Modern disk-based storage systems are not energy proportional, because disks consume almost as much power when idle (but spinning) as they do when actively accessing data. We combine a power-aware, solid-state (flash) cache and a reliability-aware disk spindown mechanism to significantly improve storage energy proportionality without hurting disk reliability, data integrity, or performance. We evaluated the resulting power- and reliability-aware hybrid flash-disk RAID storage array and found that it reduces energy consumption by 85% compared to a similar-cost, similar-performance typical configuration of all SAS drives that are never spun down. Our design also achieves almost 50% energy savings compared to hybrid flash-disk systems tuned for performance or that do not take full advantage of opportunities for safe spindown. Further, unlike most previous work that exploits spindown to save energy, we limit the rate at which disks are spun down to avoid premature mechanical failures, whereas reliability-unaware spindown algorithms can exceed manufacturer waranteed lifetime spindown limits in as little as one year. © 2011 IEEE.