Sorry for my late reply, I was on a road trip to Göteborg yesterday.
Here's some things that I want to point out before doing anything:
Use safety glasses!
The cutting disk that you wanna use is very thin (preferably .4mm) and is very fragile. If the cutter shatters, things can go very wrong!
When you make a cut, hold the machine with you're right hand (if you're right handed) and the piece that you're gonna cut with your left hand.
Start the cut at the side of the piece closest to you and work you're way outwards to the other side of the piece in a straight line.
The cutter disk should (or must) be on top of the piece that you're cutting.
When you cut, apply pressure but not too much. If the machine stops when you apply the pressure, it's not powerful enough to do the job properly.
The metal that you're cutting will get hot, so don't rush it.
The hard drive disks get scathed very easily, have that in mind.
And last of all, be patient and careful all the time.
Ok, the mirrors doesn't need to be round, a square(ish) works also. Just not ideal.
But I'm gonna show how I do:
Use a thin cutting disk.
Cut the hard drive disk in halve so it gets easier to work with.
Take a piece of masking tape and draw out the size that you want. I made two mirrors at ones.
Cut the the piece out by following the tape.
Then make some further more cuts.
At this stage you're gonna change from cutter to a sanding disk. I use that pink stone thing for support as the sanding disk is made out off paper.
Work you're way by holding it like this:
When you're done it will look something like this:
Yay, done!
Any questions?