8KNXP "SIL set" keeps needing rebuilt

I looked through this entire group hoping to find an answer but I
didn't, so here goes.....

I'm a newbie to RAID. I have a 8KNXP with twin "mirrored" Western
Digital 120GB SATA HD's. Dunno what RAID mode that is - my vendor
helped set it up. The system is rock solid except that one of my
mirrored drives has needed to be rebuilt several times. It happened
again this evening after a power outage. It happend after installing
the Win XP service pack, and after installing a Minolta Dimage film
scanner (firewire connection). I can't remember the couple other
instances where it has happened.

Anyone have any ideas why I keep needing to rebuild the "SIL set"? The
only other problem I have noticed is that Easytune says one of my fans
is not working, but I checked and they are all running.

System hardware is as follows;

Intel Pent4 3.0Ghz w/ 800Mhz FSB, 512K S-cache, Retail w/ 3yr., fan
512MB DDR400 PC-3200 DDR RAM 64x64 Samsung - quantity 2
Western Digital 120GB SATA HD w/ 7200 rpm, 8MB Buffer
Lite-On LTD-163 16X Beige Internal EIDE DVD-ROM Drive
ASUS 52x24x52 IDE CD-RW w/ FlextraLink,
ATI Radeon 9800 Pro 128MB DDR 8X AGP
Antec SX1000II MidATX 10-bay Perf. Series Case
KingWin 450W!! Dual-fan Ball-bearing PS w/ 3-speed fan, P4,
KingWin Ball-bearing 3-pin Case fan 80mm, 2.5K rpm, 32.4Cfm
KingWin Ball-bearing 4-pin Case fan 80mm, 2.5K rpm, 32 Cfm

Many many TIA.