Thanks for your answer.
I add more information as required. Yes, I have tried another microsd card, a Samsung 32GB HC class 10, and it works correctly on the 551ML. Moreover, the Sandisk Extreme microsd 64GB that gives me problems on the 551ML, works perfectly inside another smartphone (Acer Liquid E2).
The format of the Sandisk microsd is exFAT.
With "some time", I mean this: I put the microsd in the 551ML, I turn on the phone, the microsd is correctly mounted, but after sometime, say around one hour, I get a notification icon on the bar saying that the card has been suddenly removed.
Here's a picture of the problematic microsd: