Yes, " Unable to " is very different to that of " Not able to ".
Grammatically any failure / fall of action is denoted by " Un+Able ". When people speak, they colloquially tend to get the " Not " as the strong notation of affirming the failure in action, deriving " Not able ". As a layman both means the same, but per grammar the fail of action is mentioned " Unable to ".
Regards,
Subu
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