It's worth noting that 3.5" drives also use 6-32 (at least all mine do, I don't know if that changes over 4TB though)Ouch, that sucks. Are you 100% sure you were using the correct screws? It's pretty easy to get M3 and 6-32 mixed up, and they will strip if the threads are mismatched.
All the chassis threads (including the brackets) on the M1 are M3 threaded, and there are only five 6-32 screws included for PSU mounting (don't use them for anything else!). Those should be in their own labeled bag. Any other screws you get with your PSU, motherboard, etc. should not be used, unless you're sure those are also M3 threaded.