Sometimes during your second playthrough and beyond, you will get an extra SDU 
Backpack after repairing a Claptrap even after you have reached the 42 maximum 
allowed slots in your inventory, which is not supposed to happen, and is a known 
glitch in the game. Thus, before you turn in a repair kit to rescue a Claptrap, 
exit the game to save. Then, turn the repair kit in to the Claptrap. If you do 
not get an SDU Backpack, leave the game through a non-saving method, such as 
signing out of your profile or resetting the system. Then, try again until you 
get an SDU Backpack.