This is what I have read on several different bb sites. The size is dependent on the OS on the device. 4.2xxx can support up to a 4GB memory card. (the second number in the version, in this case the 2, times 2)
4.5 can support up to a 10 GB.
So in your original post/question, the 6GB was too big for the OS.
If this is in error, Please let me know.