if the HDD wasn't correctly partitioned or formatted, I think USBLoaderGX would refuse to launch the game and would provide you a reason.
The fact the loader launches the game without any warning means your device is good, the path to the iso is good, the filename is good, etc.
I think the problem was your game. Maybe it wasn't correctly written to your hdd? It's strange that it's working now if you only changed the cluster size.
Maybe the loader doesn't check if cluster size is neither 32 nor 64? I think I didn't add a check for smaller cluster size. I guess I'll fix that when I have time.