macOS Sierra External Drive Mounting Issues

macOS Sierra 10.12

MacBook Pro (Retina, 15-Inch, Late 2013) 2.3GHz, Intel Core i7, 16GB Ram

I am having some drive mounting issues since upgrading to Sierra.

USB External HD (WD My Passport Ultra)

• When it is connected and I reboot, the drive does not auto-mount.
• When already booted, and I plug in the drive, it does mount.

USB External HD (G Drive) connected to Apple Thunderbolt Display (27 Inch)

• When display is connected, drive is powered on and I reboot, drive does not auto-mount.
• When already booted, and I plug in the thunderbolt display, it does mount.

To simplify, it seems when drives are connected at bootup, the drives are not recognized and do not mount. Has anyone experienced this? Does anyone have a fix they could recommend?

Thanks!