I'm sorry,
the same thing happened to me as well a couple of weeks ago.
I have to G6real, one for me, and one for my brother(the newest one).
Mines never had any problem, although my brother's one became sorrupted in the same way you describe after only one week.
Luckily though I had just moved all his save files from the M3 to the PC and converted them online, therefore after formatting the G6real I was able to retrieve most files.
What can I tell you ? It's always a good idea to backup your save files, it's really fast if you think about it( instead of backup the whole card you just select the files you want to save ) and it can save a lot of disappointment if you get in any trouble.
I do perform backups on a regular basis, but most importantly I backup at least the save files before I try to install new homebrews.