Previous studies proposed energy-efficient solutions, such as multispeed disks and disk spin-down methods, to conserve power in their respective storage systems. However, in most cases, the authors did not analyze the reliability of their solutions. According to research conducted by Google and the IDEMA standard, frequently setting the disk status to standby mode will increase the disk's Annual Failure Rate and reduce its lifespan. To resolve the issue, we propose an evaluation function called E3SaRC (Economic Evaluation of Energy Saving with Reliability Constraint), which considers the cost of hardware failure when applying energy-saving schemes. We also present an adaptive write cache mechanism called CacheRAID. The mechanism tries to mitigate the random access problems that implicitly exist in RAID techniques and thereby reduce the energy consumption of RAID disks. CacheRAID also addresses the issue of system reliability by applying a control mechanism to the spin-down algorithm. Our experimental results show that the CacheRAID storage system can reduce the power consumption of the conventional software RAID 5 system by 65% to 80%. Moreover, according to the E3SaRC measurement, the overall saved cost of CacheRAID is the largest among the systems that we compared.