SD card MBR corrupted = GW .3DX save lost + CIA save lost + emuNAND lost + etc.
Although data rescue software may help with it, it doesn't work for 100% cases.
I don't know whether it's a perticular case or not.
But since it happened at least once, take care of it and your data.
BTW, this case happened on GW 3.5.1.
While the guy may be a Gateway user,
We should check if his card is a legit red card.
And also, if he used other CFWs.
(It'll be good to know what GW firmware he is on)
If above one of above two conditions are met,
it is either:
1. He's unlucky and he crapped out on his SD card
2. GW has some code to corrupt SD cards of not real users. (which still sucks, but not as bad as bricking)
However, even if he is a 100% legit GW user,
There is a chance some code inside crapped out (maybe SD card brick code) and corrupted his SD card.