Subfloor Sanding

Unfortunately, my supplier delivered the wrong subfloor for our 2nd floor and I didn't arrive in time to catch it, so they used cheap stuff.  Luckily, they did use Advantech on the first floor, which after many rain storms and standing water, is still flat as can be.  Unluckily, the 2nd floor cupped at all the connecting boards butting up together making it uneven across boards...

I tried renting an orbital standing sander, but that failed miserably.  So I resorted to using a high powered belt sander and was able to sand down and flatten all the seams making the floors flat and level...