Solved, but not understood
I tried that and it gave me the same error, so I tried disconnecting the RAIDed drives and just booting the SATA drive. Now, I received error 15 instead of 21. After reinstalling grub with the IDE drives removed, I successfully booted the SATA drive. After plugging in the IDE drives however, error 21 returned. At this point, I changed the BIOS boot device from HDD0 to SCSI and now it is booting properly. After having this machine for two years, I only now just found out that not only Linux, but also the BIOS, treats SATA as SCSI.