A variation on the CD fixture produces a 3.5 inch hard drive platter fixture:
Which needed just a touch of milling for a snug fit around the platter:
Tape it down on the 3018’s platform, set XY=0 at the center, and It Just Works™:
The rather faint line shows engraving at -1.0 mm = 70 g downforce isn’t quite enough. Another test with the same pattern at -3.0 mm = 140 g came out better:
It’s in the same OpenSCAD file as the CD fixture, in the unlikely event you need one.
The Smell of Molten Projects in the Morning 
In the ‘very random question’ category: doesn’t that make an awful, terrible screeching noise? It makes me nauseated just thinking about it. (But then again, I suppose you wouldn’t know… lucky you. :) )
Even wearing my power ears, the diamond point makes a soft scritching on CDs and hard drive platters.
Seeing as how Mary hasn’t come downstairs to see what’s happening, it must not be too awful, even for someone with treble in her life.