After following the instructions on this wiki in the category Claptrap Rescue Missions on how to get rewarded all 5 backpack SDUs on playthrough 2 I now have 63 inventory slots. I also have more details on the process and a few theories of my own. All things considered, however, it is entirely possible and I got all 5 SDUs on playthrough 2 to by using that method for console users. Just look at the pages I'm following and click on Claptrap rescue missions...and try it. Any questions? Just comment and I'll see if I can help.