I’d have still expected them be writable because unlike CDs, writing a magnetic field isn’t destructive. But what I meant was whether cheap blank disks used a slightly different (cheaper) magnetic alloy with a shorter read/write limit than commercial software where they maybe spent a little more knowing it needed to be read often?

I do remember cassettes had different quality magnetic strips but I don’t know if the same was ever true for disks too… so it’s probably bonkers. I’m just throwing some ideas around without much thought. :)

It might just be a case that college kid me was less careful with his disks than you were.

More recently though, I have unboxed my old collection of 3” (not 3.5” but literally 3 inch) floppies for the Amstrad CPC 464 and they almost all worked despite being nearly 40 years old. So I definitely believe you when you post about your success rate.