I think you could give it a shot with ATA Secure Erasing one of them and seeing if it performs faster. Although 4 months at 50% utilization at (say) 2GB/s is so

I think you could give it a shot with ATA Secure Erasing one of them and seeing if it performs faster. Although 4 months at 50% utilization at (say) 2GB/s is some ~10PB of I/O, so I'm not sure if I would expect what you're seeing to be a temporary slowdown...