When configured for the first time my paired 160GB WDC SATA drive could duplicate 100MB file in a mere second. Now suddenly write speed is around ridiculous 20MB/s! I tried change data/power cable, defrag, cleaned up MFT record and change SATA channel but no avail. If I split them as normal IDE, I notice that my second HDD has degraded quite a bit from ~100MB/s to ~70MB/s of transfer rate, but how come a RAID 0 make them even slower?... It's SB600 controller (of AMD 690 chipset). Yeah the driver sucks too (no WHQL)
Guess I will backup the whole array and back to AHCI mode as neither Mac OS X nor PCBSD recognized the RAID disks anyway.