Can you clarify what you mean by LabVIEW quites responding. Do you mean the rest of a program that uses the Vi, do you mean the whole development environment freezes? Can you attach a screen shot of the vi in question? If so, please make it a png or gif rather than a BMP image, they end up being huge.
Not with USB specifi but dll in general.
Check the docs for your dll and see if you can find a way to shut it down and unload it. It sounds like there is SOMETHING still running and the dll is a likely suspect. You could eleminate all calls to the dll completely to verify that its the dll and not LV causing the hang.
If I open LabVIEW and run a VI that is not dependent on the .dll file, everything works fine. But if I open a VI that depends on the .dll file, and then close the VI without running the VI, the entire LabVIEW environment ceases to respond. The freeze up problem exists whether I run the VI or not.
It sounds like there is some callback from the DLL on load.
The test would be to set up the VI the same way with a call library function node but don't configure the node for your DLL. If that doesn't crash it, but after configuring it does, then it is related to the DLL not LabVIEW. There could be something loading when the DLL is loaded when you open the VI.
To Jeff and the others who responded,
Thanks for your help and suggestions. The problem was in the .dll file and I think it is fixed now.