This could be due to several reasons, but it's probably because the appropriate driver is not configured in the kernel. Check and make sure the appropriate driver (EATA-DMA for most DPT cards) is configured.
The RAID has not been configured properly. If you're using a DPT storage manager, you need to configure the RAID disks as a single logical array. Michael Neuffer ( neuffer@kralle.zdv.uni-mainz.de) writes "When you configure the controller with the SM start it with the parameter /FW0 and/or select Solaris as OS. This will cause the array to be setup to be managed internally by the controller."
Read the SCSI-HOWTO again. Check the cabling and the termination. Try a different machine if you have access to one. The most common cause of problems with SCSI devices and drivers is because of faulty or misconfigured hardware. Finally, you can post to the various newsgroups or e-mail me, and I'll do my best to get back to you.