Western Digital 4 terabyte drive not seen in High Sierra
iMac updated to High Sierra, 4 terabyte Western Digital External Drive will not mount though 1 terabyte will.
I've had nothing but grief with WD drives and Mac (and Synology). Even their WD Utilities app can't see it!
Much better luck with HGST even though they're owned by WD.
Try checking WD's website for any updates.... they're probably trailing miles behind MacOs which is why the results are so iffy.