Snapped Bolt

A Dremel would work, but I would get someone else to hold a vacuum cleaner nozzle as close as possible to the bolt while you're doing the grinding. Metal bits on the Mobo are inadvisable...

Use a dremel grinding disc, and grind a screwdriver slot into it - you might be able to unscrew it then.

If the screw is steel, place a magnet close and use a flat swiss file. You shouldn't need to use a powered device.

Wear eye safety gear while doing it.

Whether you remove it or not isn't important to the hdd/etc. Slot it & unscrew or just file/grind it flat. If it's a rust hdd I'd not want to apply significant vibration. Or just mount it the other way, screwing into the base of the hdd etc, either in the 5.25" bay or elsewhere.

Generally, grinding on a chassis, just does not mix with fine pitch electronics. You remove all the electronic bits, before doing chassis mods, so you can vacuum out the metal filings as well as wipe down the chassis until it's clean again.

It's hard to believe, that's all the screw types in the case, because at least one screw seems to be a close fit but the wrong item. It might be a couple of screw types associated with U-rail adapters for hard drives in optical drive bays. If you believed these pictures, nothing could ever go wrong.

Thank you, that's useful for the future :-)

This particular screw seemed like a fine threaded HD screw but turned out to be even smaller. Mind you I have just completed a new build with 3 x NVMe which came with the proper screws. However, they are different to the screws for the PCIe boards the NVMes were fitted to in the old PC, each of which in turn was different to the other!

There are US threads and metric threads and you need to stock both. Its likely that the snapped screw is in fact M3, somewhat smaller than the #6-32 UNC that is commonly used to fasten most of the case metalwork.

A lot of stuff is starting to use M2.5 now And M3.5 is in use as well.

I now have boxes of all of the above and taps and dies for them too

I was recently hit by a Dremel disk shattering, no damage but made me think. I now keep safety glasses by the Dremmel, for those just 1 second jobs.