Had no issues with guid, no error etc, same with setting up a FAT32 HD and moving games onto it.
However WiiFlow just hangs on the WiiFlow logi with black background. My Wii drive flashes blue and it just sits.
if I disconnect my HD or put it top USB slot it starts, just not finding games as expected.
Drive and games are fine I think. USB loader GX (or whatever) sees them and plays just fine.
Black Wii running 4.3U
* Update *
I updated to WiiFlow 4.0.1 and its is working now.