With recent technological advances and continuously decreasing cost per GB, flash memory based solid state disk (hereafter, flash disk) has already been considered to be the future replacement of rotating-based hard disk. Unlike hard disk drives, flash disks are less prone to mechanical wear and tear, have no seek or rotational delays, and consume much less power. However, flash memory has limited write endurance as a block becomes unreliable after a finite number of erasure/write cycles. This characteristic hinders flash disks from being readily used for server domain. Consequently, wear-leveling techniques are employed to distribute block erasures and re-writes evenly across the medium. Existing wear-leveling techniques are essentially intra-disk data arrangement and distribution schemes, which organize data within one flash disk. Nevertheless, an inter-disk wear-leveling technique, which can ensure uniform wear-out of blocks across multiple flash disks, is much needed for enterprise-class storage systems where flash disks are organized in a disk array. In this study, we propose an inter-disk wear-leveling strategy called iWell (inter-disk wear leveling) that dynamically and evenly distributes writes across a flash disk array. The proposed iWell technique compliments existing intra-disk wear-leveling techniques to further improve flash disk's reliability while achieving a better performance in most scenarios. Results from a comprehensive simulation study based on both real-world traces and synthetic benchmarks demonstrate that iWell significantly improves the reliability and lifespan of a flash disk array by largely reducing the variance in terms of writes and merge operations among disks.