@FastFreddy 

Command IS correct, you don't have a space before the '-'. However, you do have SMB 1 running it seems.

At this point I suspect the USB drive.

Quite possible it was corrupted when you disconnectedx it from the R8000 (on its web page you have do the 'safely remove' and even on the TP-Link page as well) before disconnecting it otherwise corruption can happen.

So, take it to a PC and see if it works there. A corrupted drive will not mount usually and need to have CHKDSK run on it.

What IS the FILESYSTEM on that drive. NTFS works best.

Is the drive SELF-POWERED? That means it has a Power Supply to supply the power? A 'Portable USB drive' takes power from the USB port. Some routers may not have enough power to supply all that is needed, hence the drive will not work.