A Storage Deck Upgrade (or SDU) is an upgrade item that can be applied to a character's inventory to increase that character's storage capacity. The different types of SDU that players will interact with include a Backpack SDU and a number of different weapon ammunition SDUs. Another class of SDU is referred to as a Weapon Equip Slot SDU is automatically applied to a character without the need for player interaction.
The Backpack SDUs each offer 3 additional slots of storage capacity. 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.
Once players complete the story on a first playthrough the higher difficulty second playthrough will allow them to repeat the Claptrap Rescue Missions for more Backpack SDUs. Unlike the first playthrough, only the first five of these missions (Safe House, The Lost Cave, New Haven, Tetanus Warren, Earl's Scrapyard) offer a chance of getting an extra SDU when completed. This can take several tries from either exiting without saving or loading from a backup file because the SDU reward is not 100% guaranteed. These additional Backpack SDUs are often referred to as glitches by players in Borderlands community forums. Gearbox has not released any information confirming the additional SDUs to be glitches and they appear to be intentional 'bonus' SDUs that players have a chance of obtaining. The appearance of the intentional nature of Playthrough 2 SDUs is the PC-confirmed program code that deliberately increases the chances of obtaining these 'random' SDUs if characters have less than 42 inventory slots already, or has more inventory slots filled when turning in the mission for the reward. Not completing the missions in the first playthrough does not affect the chance of getting an SDU on the second playthrough.
The original total maximum inventory slots is 57 on the game disc alone, but with the release of the Zombie Island of Dr. Ned DLC, which includes an additional Claptrap to save, backpack capacity can be increased to 63. The two additional SDUs can be gained by accessing the DLC from each main game playthrough (one per playthrough).
In The Secret Armory of General Knoxx the Claptrap in Lockdown Palace has a chance to reward a Backpack SDU in Playthrough 1 (much like playthrough 2 in the regular game), bringing the latest inventory maximum to 66. Playthrough 2 has not yet been seen to reward another backpack upgrade.
Furthermore, Claptrap's New Robot Revolution features an additional guaranteed 2 SDU's to collect throughout the two playthroughs, increasing the total possible inventory space to 72 slots.
Upgraded Backpack Storage Capacity
First Zombie Island
Second Zombie Island
Secret Armory of General Knoxx
(Playthrough 1 -or- 2, not both)
First Robot Revolution
Second Robot Revolution
Claptrap Rescue Missions
Weapon Equip Slot SDU
Two Weapon Equip Slot SDUs are available as required mission rewards in Playthrough 1. Together these increase the weapon equipping capacity of a character up to four weapons, allowing a greater selection of weapons in the thick of a fire fight. Each weapon equip slot provides self-contained 'storage' of one weapon in addition to the inventory capacity.
The two missions that offer these as rewards are:
Eight other SDU types that players interact with are SDUs for increasing a character's ammunition storage. These SDUs are purchased from Ammo Vendors and have quality levels that increase in the same manner as weapon quality, indicated by colour. Characters gradually unlock higher quality weapon SDUs as they increase in level and progress through the game. The higher the quality, the greater the capacity and the higher the cost.
Ammunition SDUs do not have to be bought in the order that they appear in game. Higher capacity SDUs can be purchased later in the game without having to be added to the low capacity SDUs at the start. Consequently, a cost effective method of upgrading all of a character's ammunition limits is to focus on upgrading just the few that are mainly used by the character, then waiting until the dark orange quality SDUs are available before upgrading to the maximum ammunition capacity for all weapon types. Players with The Secret Armory of General Knoxx DLC can purchase orange ammunition SDUs at T-Bone Junction for $223,286 at any time. They can either buy the dark orange SDUs at the vendor at Crawmerax' lair for $722,393 in Playthrough 1 or join a game on playthrough 2 and purchase dark orange ammunition SDU at T-Bone Junction for $722,393.
Return on investment
Not all SDUs grant the same total ammunition capacity bonus, in terms of percentage. For example, Repeater Pistol, SMG and Combat Rifle SDUs will give a 50% boost per level, reaching a total of 400% base capacity. Shotgun and Sniper SDUs though, will only give a 25% boost per level, reaching a total of 250% base capacity. Revolvers, Launchers, and Grenades gain 33% base per level, ending with 300% base capacity.
Skill Point SDU
Mad Moxxi's Underdome Riot
Upon completion of the Prove Yourself mission in Mad Moxxi's Underdome Riot, characters are rewarded with a Skill Point SDU. Much like Ammo SDUs, the Skill Point SDU is automatically applied to a character giving one additional skill point for completion of the mission line. This is awarded in both PT1 and PT2.
Clap-Trap's New Robot Revolution
Upon completion of Helping is its own reward... Wait No it isn't! in Robot Revolution, characters are also rewarded with a Skill Point SDU. This is awarded in both PT1 and PT2.