I was hoping someone else with a Z might try and answer this one, as I have yet to corrupt a SD/MMC card on my Z, but a couple of things to try.
My 5600 doesn't seem to like it when I use /dev/mmcda to access the card, try using /mount/card instead. Might make a difference, then again it might not.
If you have any other devices that can format a SD card, like a card reader or a digital camera, try formating it back to FAT16/32. Some of the cards flake out when they're formatted as ext2.
If that doesn't work, I'd return it. An SD/MMC card shouldn't bite the dust after 1 usage. Try a SanDisc card. I got a 256Mb card that's been working pretty good over the last couple of months.