Even the 12TB drives are only advertising 250MB/s. https://www.anandtech.com/show/11903/seagate-ships-consumerf... That's a lot higher than the 150 MB/s or so you'd get from a 3TB drive, but it's not pushing the limits of SAS or SATA. Unless you cheaped out and connected a bunch of drives to a single controller.

SAS 4 is the works and should be finalized soon. It will supposedly have 22.5 Gbit/s (2.8 Gbyte/s) of bandwidth. Whether or not a HDD can actually saturate this is suspect.

why? do you expect mechanical drives to deliver >600MB/s peak transfers?