How to reclaim all the space on a NAS RAID drive for us as a regular Mac drive
My (maybe educated) guess:

Your docking station (StarTech SATADOCK22UE) has a useless/faulty/old USB Bridge chipset firmware and you are hit by some double 2.2 TB barrier (=32bit block addressing error).

Depending on the firmware of the WD60EFRX it has a size of 6000527425536 B - 6001424400384  B (ref).

The 2 x 2.2 TB gpt barrier is 4398046511104 B. Disk size - 2 x 2.2 TB gpt barrier = ~1.6 TB!

Depending on the firmware and the real size of the disk you will only see ~1.6 TB in macOS.



Solution: get another external case and mount the 6 TB drive in there!