http://www.highpoint-tech.com/HPT368のメーカーサイトです
んで、ここのFAQに
Q: What does the RAID 1 "Mirroring" feature do?
A: RAID 1 (Mirroring) configures an array of 2 drives to write duplicate data and read in parallel. The HPT368 performs reads using advanced data handling techniques that distribute the workload in a more efficient manner than using a single drive. When a read request is made, the HPT 368 selects the drive positioned closest to the requested data, then looks to the idle drive to perform the next read access.
Q: How do I use the RAID 1 "Mirroring" feature?
A: Make sure that the size of the source disk is less than or equal to the size of the target disk before trying to perform the RAID 1 "Mirroring" feature.
と書いてありました
ん〜、ぜんぜん助けになってないな…