If that motherboard is more recent than Debian Stable, the included Linux kernel might not support the C621A chipset and/or other chips on the board. Please check if the Linux kernel in Debian 13.6 has support for the Intel C621A chipset. Also check if there are bug reports for that combination. If support is lacking, or if there are issues, you might need a newer kernel from backports or you might need to run Testing.
Since it's a software RAID, I wonder if this might be the issue. I'll check the driver support to see what is in the kernel and also if there are proprietary drivers that Intel provides.