Don't ask me why, but I did an experiment where I tried to remove the magnetic iron oxide layer from a floppy disk by soaking it in HCl for a week. I put pieces of floppy disk in 0 - 1 M HCl, then measured their absorbance at 700 nm. HCl concentrations at 400 mM or more were able to remove about half of the absorbance, but not more than that.
This suggests that something is preventing the acid from dissolving the remaining iron oxide, and that simply increasing the concentration beyond 1 M won't help. However, I can still scratch the surface and remove some of the color, suggesting that it isn't buried throughout the plastic film, but still accessible on the surface. But I don't want to spend forever scratching the iron oxide off floppy disks, so I would still like a simple chemical method to remove the metal. Does anyone have any suggestions for removing it?