ChillyWilly, thanks for your suggestion. I tried it in all USB ports, but LED is always solid red light and software does not recognize slim loader is connected.
Windows pops up USB Device Not Recognized when I plug it in and then says Unknown Device installed successfully and ready to use...
So I also tried installing drivers again from NEO2/Driver/XP_or_Later folder, but it did not solve the problem.
I never encountered this solid red light before, I guess Slim Loader is just broken; PCE Flash cart is still working fine--just I can't reflash it without slim loader!