And now I know exactly why... but I don't have a solution.
It looks like that there is some kind of process scanning the microSDXC card when the phone is booting (the process that scans the card to make sure the state is ok)... and I'm getting the "modified" status right after that this process has finished his job.
If I remove the card before booting the phone... and insert it when the phone has finished booting... the "normal" status will be shown.
And I'm not the only one with this problem... http://forum.xda-developers.com/show...1810450&page=2 (I just found this thread)
Now is there a way to get in touch with Samsung to eventually get this problem fixed?