There are many possible, may be single or multiple. If shorter 8087-8087 cable available soon, just waiting it for testing.
If suspect HBA problem, could you use reverse 8087-SATA cable to connect BP to MB for test. ( you already have )
If suspect BP problem, could you use stright 8087-SATA cable to connect HBA to HDD.
So two different kind cable could help troubleshoot most parts, I always have that and ready for test need. All my HBA, Expander, 10GNIC, SAS cable .... come from china, may be fake, old pull whatever .... just well test would be fine. Usually they are new, I won't expect it is orginal as this price.
Test plan also importance, to avoid loopping ( I think you in that ), you may need divide whole system in small part, i.e. test HBA, test 1st BP, test 4 HDD ...... and may not only use Unraid for shooting, the main goal was as fast as found out the failure point.