grep dmesg and see if the link is at sataII speeds, if so its almost certainly the cable. If not, still might be ok. Your motherboards IC may perfer some sata ports over others when its at its limit. Remember it likely runs on a 1x pcie bus back to the system bus which is ~ 800MB/s. You have 8 drives so...
Only panic when there isnt a lot of data being pushed and the drive still performs badly and you have replaced cable
grep dmesg and see if the link is at sataII speeds, if so its almost certainly the cable. If not, still might be ok. Your motherboards IC may perfer some sata ports over others when its at its limit. Remember it likely runs on a 1x pcie bus back to the system bus which is ~ 800MB/s. You have 8 drives so...
Only panic when there isnt a lot of data being pushed and the drive still performs badly and you have replaced cable