I have some rectangular blank CD-Rs the same size as business cards. When used as CD-Rs they hold about 120MB. They're mirror-shiny (with a prism rainbow across it) and have a small spindle hole in the center. They came in a plastic slipcover.

Can anyone see any drawback to using these as signal mirrors, ones that can also hold a data backup? Would the spindle hole be good enough as a sight hole? What obvious problem makes them a poor choice for including in an EDC kit that I'm missing?