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

It thinks that its out of write cycles, however I pushed a 2TB samsung 980 pro to 7PB and it was fine, so ymmv. It did get slower after a while, but not unbearably so. I have been at 0% for 2+PB.

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I only use it for irreplaceable data that I only have one copy of though.