There could be compatibility issue here as WD does not seem be listed in BE 16 HCL. However, is there any other USB storage you can try to configure as disk storage in BE to confirm it works and gives issue only with WD 8TB USB store you are trying to add?
You may also want to run B2D Test tool (article 000037869) against this WD device to verify it's functionality level.
You might be dealing with as issue related to sector size. I remember this years ago when disks got big.
As previously suggest try with another disk, even a thumb drive, to see if BE can configure that OK.