The High-Low Water Mark destage (HLWM) algorithm is widely used to enable a cached RAID5to flush dirty data from its write cache to disks. It activates and deactivates a destaging process based on two time-invariant thresholds which are determined by cache occupancy levels. However, the opportunity exists to improve I/O throughput by adaptively changing the thresholds. This paper proposes an adaptive HLWM algorithm which dynamically changes its thresholds according to a varying I/O workload. Two thresholds are defined as the multiplication of changing rates of the cache occupancy level and the time required to fill and empty the cache. Performance evaluations with a cached RAID5 simulator reveal that the proposed algorithm outperforms the HLWM algorithm in terms of read response time, write cache hit ratio, and disk utilization.