I had here an old Dell server with a failed RAID array – the drives were completely unresponsive, and every diagnostics tool came up clean. So all you can do is pull the whole system apart piece by piece until you get somewhere.
Thankfully life came back, possibly the PSU just can’t handle the juice, since things started working well once an unused SATA expansion came out. One of the two drives was now being reported as degraded, so it wouldn’t join the array. A quick verify and the Adaptec tool then allowed the array to be rebuilt.
I love it when it all works out in the end.