These additional Backpack SDUs are often referred to as glitches by players in Borderlands community forums.
This can take several tries from either exiting without saving or loading from a backup file because the SDU reward is not 100% guaranteed. Unlike the first playthrough, only the first five of these missions (in the Sledge's Safe House, Lost Cave, New Haven, Tetanus Warren, Earl's Scrapyard locations) offer a chance of getting an extra SDU when completed. Once players complete the story playthrough 1, the higher difficulty playthrough 2 will allow them to repeat the Claptrap Rescue Missions for more Backpack SDUs. In summary after completion of playthrough 1 and the DLCs there is a guaranteed total backpack capacity of 48, with a maximum possible capacity of 51 (if Claptrap Rescue: Lockdown Palace provides a backpack SDU). Completion of Claptrap Rescue: Lockdown Palace in The Secret Armory of General Knoxx has a chance of rewarding a Backpack SDU in playthrough 1 and is not guaranteed.Ned, The Secret Armory of General Knoxx and Claptrap's New Robot Revolution DLC there are an additional 3 Claptrap Rescue Missions to complete. With the addition of the Zombie Island of Dr. Each one is gained as a reward for completing one of the ten Claptrap Rescue Missions, for an extra thirty spaces of storage that can be earned throughout the game to a total capacity of 42 on the first playthrough. The Backpack SDUs each offer 3 additional slots of storage capacity. 3.2 Borderlands 2 and Borderlands: The Pre-Sequel.