Something you must know before starting.

Make sure when playing borderlands you can quit without saving. Now, you can learn how to Read Only farm, or simply Quit Without Saving farm Here

These claptrap rescue missions always involve finding a damaged claptrap then finding a repair kit and then going back to it for a chance to get a backpack SDU or grenade mod, XP and money. Sometimes the claptrap will even help you by locating a chest after turning in the mission.

Now when you first start playing you will be in playthrough 1. In this playthrough all the claptrap rescue missions you do will give a 100% guranteed backpack SDU as a reward except one.

There are technically 14 claptrap rescue missions in total (in each playthrough). Although the first one you do is part of the story but doesen't give you a backpack SDU as a reward.

The mission Claptrap Rescue: Lockdown Palace for some reason doesn't give you a 100% chance of getting a backpack SDU as a reward, instead sometimes it may give you a grenade mod. So by using the exit before saving method keep trying until you get it. By having a full backpack or even overloaded by mission rewards the chances of getting the backpack SDU seems to be higher but this still hasn't been proven properly so you don't have to do it. Make sure you don't walk near a save point after you get a grenade mod since the game will save and you won't be able to ever get that backpack SDU from that particular mission.

This is the List:

Mission names, locations and backpack SDU availability.
Claptrap Rescue in Fyrestone - No
Claptrap Rescue: Safe House in Sledge's Safe House - Yes
Claptrap Rescue: The Lost Cave in Lost Cave - Yes
Claptrap Rescue: New Haven in New Haven - Yes
Claptrap Rescue: Tetanus Warren in Tetanus Warren - Yes
Claptrap Rescue: Scrapyard in Earl's Scrapyard - Yes
Claptrap Rescue: Krom's Canyon in Krom's Canyon - Yes
Claptrap Rescue: Trash Coast in Trash Coast - Yes
Claptrap Rescue: Old Haven in Old Haven - Yes
Claptrap Rescue: The Salt Flats in The Salt Flats - Yes
Claptrap Rescue: Crimson Fastness in Crimson Fastness - Yes
Claptrap Rescue: The Lumber Yard in The Lumber Yard - Yes
Claptrap Rescue: Lockdown Palace in Lockdown Palace - Yes but not 100% chance
It's a Trap... Clap in Tartarus Station - Yes

Now things are very different from playthrough 1 since only 7 (8 if you haven't done Claptrap Rescue: Lockdown Palace in the first playthrough) of the claptrap missions give you a chance of receiving a backpack SDU or a grenade mod (that we don't want).

First thing you must not do is complete the base game story in playthrough 2 (killing the destroyer), as this will give you a 0% chance of obtaining a backpack SDU or even a grenade mod in Claptrap Rescue: The Lumber Yard and It's a Trap... Clap. Make sure you do these two missions before doing Destroy The Destroyer as I made the same mistake!

This is the list:

Mission names, locations and backpack SDU availability.
Claptrap Rescue in Fyrestone - No
Claptrap Rescue: Safe House in Sledge's Safe House - Yes but not 100% chance
Claptrap Rescue: The Lost Cave in Lost Cave Yes but not - 100% chance
Claptrap Rescue: New Haven in New Haven Yes but not - 100% chance
Claptrap Rescue: Tetanus Warren in Tetanus Warren Yes but not - 100% chance
Claptrap Rescue: Scrapyard in Earl's Scrapyard Yes but not - 100% chance
Claptrap Rescue: Krom's Canyon in Krom's Canyon - No
Claptrap Rescue: Trash Coast in Trash Coast - No
Claptrap Rescue: Old Haven in Old Haven - No
Claptrap Rescue: The Salt Flats in The Salt Flats - No
Claptrap Rescue: Crimson Fastness in Crimson Fastness - No
Claptrap Rescue: The Lumber Yard in The Lumber Yard - Yes only if playthrough 2 story is not completed
Claptrap Rescue: Lockdown Palace in Lockdown Palace - Yes if not completed on playthrough 1 but No if already done on playthrough 1
It's a Trap... Clap in Tartarus Station - Yes only if playthrough 2 story is not completed

If you did make the mistake of completing the story in playthrough 2 the only way of definitely getting the backpack SDU from those 2 missions is to go on a friends hosted game in which they did not complete the main story on playthrough 2.

The legit max inventory space is 72.
An inventory of 42 spaces unlocks an achievement called "Fully Loaded".
Each backpack SDU gives 3 additional spaces to the inventory.
With more space you can have a larger collection of your weapons you would like to keep. With the bank from "Mad Moxxi's Underdome Riot" the total is 114. With the weapon, shield class mod and grenade mod slots this brings this up to 121.

The odds of getting the SDU (if they're not guaranteed) increase if you are full and especially if you are overloaded from mission items. As well as if the player has less than 42 slots available - Thanks Bloodscape for the info

If you want to know more about this topic, you can acess The Wiki [borderlands.fandom.com]

If you have any questions, simply comment below.
What would be very helpful is if anyone can spot any spelling, punctuation and grammar for me to fix.

Hopes this helps and enjoy playing one of the best games ever!