You can't get paid upfront and at the same time get a liability waiver. For a 100% guarantee with full liability $30k doesn't actually sound ridiculous because it would require obtaining 100% identical hardware and doing at least one test run on that hardware before actually doing it on the production hardware. What the contractor did is basically "wing it", explain a way to get zero downtime to the client and then not actually offer a guarantee by doing the operation straight on the production hardware. Really this was more about convincing (ie bullshitting your way through) the client to let you do the work than actually doing it properly and for a huge sum of money. It wouldn't surprise me if there was actual downtime for a few seconds and the client simply didn't notice it.