I don’t think your HDD can achieve this bandwidth and based on e.g. these newegg specs you could get ~255MB/s:

this 3.5-inch drive features 8TB capacity, 7200RPM spinning speed, SATA 6 Gbps host interface and 256 MB Cache, and delivers sustained transfer rate of up to Up to 255 MB/s (for reference only)

You could also profile the read speed to check the “for reference only” claim and might see a lower performance.