Having some trouble with Sanding hardwood floors
It's hard to tell without pictures or exact model number, but based on your comment you've probably rented a buffing (finishing) floor sander not unlike this one. For very uneven floors you should have indeed started with a (flooring) belt (or drum) sander. Belt/drum sanders (flooring or otherwise) can remove a lot material quickly.

Even Home Despot's own video recommends you get a drum/betl sander for solid wood (as a start). The orbital sander they rent is for finishing. They say nothing about their random orbital, and I suspect it will work by itself, but slowly; you'll probably have to change discs 4-6 times (in each pass) for the entire surface you have to make steady progress. I can't tell from the pictures how uneven your slats are or how much extra time (and sanding discs) that might take.

Dealing with the height differences is a little tricky even with a drum/belt sander: you'll probably want to sand at 45 degrees initially so the sander never rests on a single slat (or entirely misses the adjacent one), then go with the grain for the subsequent passes. I'm guessing you can achieve the same effect as 45-degree drum sanding with a random orbital by hovering long enough in an area until it levels itself. YMMV.

A NYT article on the topic also has this interesting titbid:


  Some online sources recommended the U-Sand sander [which seems to be a random orbit sander with four discs -- my note], which theoretically eliminates the need for a drum sander and a buffer. The manager of my local rental agency tried to talk me out of it, saying seven out of 10 people trade it in for a more powerful drum sander.


And the reporter also had issues with the U-Sand: even after using it for four hours he couldn't get to all the "valleys" in his floor. So unless you have oodles of time at your disposal, a random orbit sander is probably not the best idea for a floor that's pretty uneven at the start.

Clarke has quite a bit to say about their multi-purpose random orbital  (which just as the U-Sand is intended to replace both the belt and edger) but what I read into that marketing (especially if you compare to what they say about their drum and edger) is that it's a gentle machine wiht low risk of wrecking floors in the hands of the untrained, but in the subtext that also says it's rather slow in removing material or making progress with uneven floors. The same user experience with respect to dips/valleys has been reported for the Clarke random orbital as well.