Why is BIOS switching from IDE to RAID mode?

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http://superuser.com – Acer Aspire M3203. BIOS will randomly default HDD interface to RAID mode and fail to boot Windows 7, claiming there is no RAID array defined and, subsequently, "no boot device." If I go into BIOS and change it to IDE mode (options are IDE, RAID, and AHCI) it will boot just fine for a few times again. Even when it does successfully boot in IDE mode, it still stops somewhere during POST to "scan for storage" and report "no defined array" until chugging along. The system has never had any RAID array set up. It's an OEM setup job, 2-3 years old. The issue seems to have begun after an AC power fa (HowTos)