Richards makes a nice 4" angled head scraper with a 12' handle and it's scrape-city.
Nailed it right on the head.
Done this many times to remove the black underlayment.
The hand scraper is the way to go.
Electric styles don't work as well. And they cause far more severe damage to the sub floor.
Lots of guys use the 3/8 plywood. But I don't recommend it for most cases.
What flooring are you putting down over the top of the sub floor?
I personally usually scrape everything and then sand/ refinish the existing flooring.
Hope my two cents helped a little.