So my friend got twI identical HDDs last month, and when he decided to run benchmarks using HD tune he noitced the maximum transfer speed is like there's 10 megabytes per second difference btw the harddrives, also he noitced the non os hdd is slightly slower then the os one, by the way both hdds use different firmware revision.
WD Red Pro 4TB NAS Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, CMR, 256 MB Cache, 3.5" - WD4003FFBX
WD Red Pro 4TB NAS Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, CMR, 256 MB Cache, 3.5" - WD4003FFBX