Hello all. I have been able to connect NanoVNA-App to my H4 via com 6 just fine. But when I place my NanoVNA-H4 in DFU mode, Com 6 goes away and NanoVNA-App says there is no DFU device connected. What am I missing here? Thank you!

Sounds normal. If Windows, have you loaded the DFU mode driver (It's not native in Windows) and as you have noted, DFU mode does notuse a COM port.)

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See the instructions here and see if that helps. https://groups.io/g/nanovna-users/files/Miscellaneous/DFU-mode%20Driver%20for%20Win10Win11

That did it Stan! I had used a similar process for DfuSe. I didn't realize that it would work for Nano-VNA-App. The STM Bootloader kept responding with "You Already Have the Most Current Driver Installed". Your process allowed me to switch it to "STM Device in DFU Mode". I can't thank you enough Stan. For this and all of the other replies!