I hate the FDS. It's unreliable, with its belt wearing out and other errors happening. With that said, I own several FDS's and many disks -- though I hardly play them. And yes, a disk can demagnetize and become useless, unless you know how to write the game back onto the disk or know someone who can do it for you.

All of this doom & gloom about Famicom Disk System...I question how you obtained your FDS.  I've owned my AV FC & FDS for over a DECADE and its still running on the same drive belt eversince ( knock on wood ). I do have to say both my AV FC & FDS were brand spanking new when I purchased them. I am considering to purchase some spare FDS replacement belts from Ebay - JUST IN CASE

Btw, i've never used "C" batteries on the FDS -  I use an AC adapter instead.