Hello, I am testing a 1 Go SD card with a Mega2560 and all the examples sketch of the SD running good execpt the read write one wherease it has working good yesterday.
It cerate the "test.txt" file but can not writing into and even if I format the card with the soft you have mentionned on the top of the storage forum, it does the same thing, what could be the problem?
I have solved the problem alone (this forum community is not very active).
The problem was that I have to connect the two ground wires of my SD module for it works.
And it can also explain I had sometimes corrupt files created that I had to format the card.
Maybe could help someone if he/she(less propable) has the same problem.
I'm active 
and so are others.
Did you read this ? http://forum.arduino.cc/index.php/topic,148850.0.html
I prefer to see a full sketch (or a complete small test sketch). Sometimes I try that sketch on my Arduino if I can't spot the problem by looking at it.