I would actually think more than backbreaking work, there'd be a lot of mind-breaking druggery work involved: stuff that involves long hours and lot of boredom. I can probably put a lot of danger in there too.
I've thought about this in my "slaves of the future" posts I've posted at various times on this board in the past. I won't go over them again.
When I think of slaves / indentured workers / prisoners put to work, I would imagine in a futuristic terraforming prison-world, they actually wouldn't be ignorant beef put to work like traditional slavery.
For one thing, I'd imagine they'd have access to tools, even tools that'd seem like they'd make good weapons. Your reluctant populations would instead be controlled by access to items required for further life, for instance oxygen or food. They might have some sort of slow-acting toxin they have to get an antitode for periodically if you're feeling sadistic.
They'd be somehow "force-taught" technical skills. Some sort of high tech method that injects knowledge direct into the brain through chemical means or some sort of sleep teaching. Perhaps psionics if you have psionics. The method would be cheap, quick, and reliable. One can imagine a tired, bored, Zhodani psionic mercenary just walking down a long line of chained up prisoners, putting his hand on forehead of each one, followed by a scream and loss of bladder control by the prisoner. Worker comfort isn't a big deal - workers might suffer from periodic headaches for months or years afterwards.
The world might have a breathable atmosphere or not. This operation would have a lot of old, second-hand (at best) equipment being used for process, stuff that's been used before. It's dinged-up and worn, and prone to breakdowns, but it's field-repairable in many cases.
In case you're wondering, that's the purpose of the workers - they actually are slaves to the machines. The machines are the ones who are actually doing the work of terraforming the world, the humans are there to do what they can do pretty well and cheaply - fix the machines. The machines and the deadlines for fixing the machines are more important than anything else. The humans are sent out there to fix a device anytime it breaks down, regardless of when or where.
It doesn't matter if a huge earth-shifter fell 200m into a ravine during a hurricane-level storm. A team of workers are sent out there in the equivalent of a range truck. They're to inspect the earth-shifter, and attempt to fix it. If they cannot, they're to salvage what they can from it for future repairs. That there's continuing danger of landslides, flash floods, or whatever isn't such a big deal; it's relatively cheap to get more prisoners. It's the equipment that's expensive to buy and ship. Similar situations might arise with gas catalyzers that are on the verge of failure, perhaps even explosion. A team is rushed there to try and save the equipment. If it explodes, well, that's too bad. If the team fixes it, great!
Then there's soul-killingly boring work like going out and following the huge armies of machines around. The purpose is to climb aboard each huge machine and inspect each one, from top to bottom, ticking off a checklist to ensure the machine isn't about to fail in some way. Other technicians would go on long, lonely circuits of atmospheric sampling machines dotted all over the southern hemisphere to inspect them, record their information in a notebook (as a backup to their regularly transmitted information, just to make sure as long as someone's out there anyway).
Those huge algae pool stirrers mentioned in an above post? Machines would definitely be better for that task. However, someone has to come by and fix the ones that break down as well as take apart the ones that are working and inspect them for damage. That's human work. At the same time, perhaps they have to take samples of algae and check it via a simple chemical test kit at different depths to ensure no mutations or anything like that are occurring.
Of course, there's also "rescue" work. Given the dangerous conditions, workers often get into accidents and such. Other teams of workers are sent out there to retrieve the wounded who aren't beyond help. Those who are hopeless can just be left there. Regardless, the vehicle they went out there must be salvaged for useful spare parts, siphoned of fuel and oil, the spacesuits stripped from the bodies to be reused, and so on.
