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By the sound of it, you have done a lot of tries to get this one Montana 610 working with your computer.
Is your computer a Windows PC? if so some other things can be tried.
Does your computer have other USB ports? if so, have you tried the Montana, patiently and waiting 5 mnutes at least after connecting it, for it to be recognized?
If you have not tried all that, please do, at least once. USB ports are not all actually equal in operation... it can help to try a different USB port. Preferably one that is fairly distant from the other USB port it failed to work in previously.