hmm, this problem is really weird :
i took another SD card, deep formated it, then put the official SD content (pfm3_SD.zip) on it.
With this SD, i could save every kind of files without any problem, ... but not the config
if i follow the action in debug mode, it crash on line 89 in ConfigurationFile.cpp
well, i can still edit the SETTINGS.TXT by hand, so it's not such a big deal