Same issue, except that my card is Class4 MicroSDHC 32GB.
Problem occurred when my I had to reboot my Android phone (unstable custom ROM) and after that the card has been practically unwriteable. Have tried several tools, repartitioning and disk checking tools - none of them complain about unfinished formatting, errors or write-protection.
Haven’t yet tried it in card reader, because it is currently lost, but I guess that won’t make a difference anyway.
Anyone else besides bobmalcolm had success with contacting and getting a replacement? For Sandisk cards, 5 years is the standard warranty.