Instructions
Locate broken Claptrap units while questing in "Borderlands." Broken Claptraps offer a mission that rewards the player with a backpack SDU. When equipped, the SDU adds three more slots to your backpack.
Stand next to the broken Claptrap unit and press the "Square" button to speak with it. A dialog box will appear. Press the "X" button to accept the mission.
Locate the repair kit item and press the "Square" button to pick it up. Your navigation icon at the bottom of the screen will show you the location of the repair kit.
Return to the Claptrap to turn the mission in. Press the "Square" button to speak with the Claptrap, then press the "X" button to turn in the mission and repair the Claptrap. You will receive an SDU in your backpack as a reward.
Press the "Select" button to open your character menu, then press "R1" to select your backpack.
Scroll through the list of items in your backpack, locate the SDU and press "X" on it to equip it and upgrade your backpack.