@ArcherC8 Yeah, I scoured the forums and none of their solutions worked..... there's ONE I haven't tried yet and that's to reformat the drive and try again.  Currently copying the data from the drive to another external and then going to try reformatting from router.

For anyone else who is scouring the internet to get their TP-Link Router & USB drive to function as a NAS...... I have a WD HDD and the solution that finally got it to work was to do a quick format in Win10 and then reconnect it.  Assuming it's an incompatibility with the WD preinstalled software... but format it and then you should be good to go.

@mike_d One more thing to add here: my set up only worked when I formatted as NTFS. FAT32 did not work. 

@mike_d  i experienced the same issue with my Tplink AC4000. It was resolved by disconnecting then reconnecting the USB drive from the router.