Perhaps your disks became magnetised by placement near speakers, tele's and actually - lots of instances in your own home that are naturally magnetised by earthen means. Oh, power boxes and any place where ground straps might be can kill magnetic media as well.
And this has been brought up before and you're probably aware of it, but many have complained about formatting HD disks as DD and experiencing all sorts of problems down the road. In the 22+ years of messing around with both 3-1/2" formats, I have not had such bad luck. Guess it might depend on the brand used?
