The problem is a common one and that is the sd card not making contact properly so to put it right you need to make sure the metal clip is pushing the sd card down properly, try lifting the clip up and pushing the centre to make it a tighter fit and make sure you get the 2 lips in place under the plastic when you push the clip down to secure the sd card.

I have seen this problem twice so far on different 900's and same cure each time, also random "corrupt" in the file manager occurs because of this problem.
Make sure the contacts on the sd card and device are clean.

Hope this helps.