Aha ok, one of the small M8 bolts which hold the caliper to the carrier? or the bog bolts which bolt the carrier to the knuckle?
I had a similar
problem with my old Audi A4 B6 - tried doing a rear brake job when I first got the car but couldn't undo the carrier bolts - so I figured out I could change the discs without touching the carriers! Well I re-fitted everything to find the disc binding on the carrier! WTF! turned out the corrosion from the Ali knuckle had grown so much it was skewing the carrier - into the new discs. The old thin disc didn't contact. That was why the bolts were so tight - as the Ali had grown stretching them! Had to make a tool to go in my big bertha breaker bar to get the carriers off to file their mounting flat.