Internal slot is picky.

My first one is kingston 8Gb class4.There is no problem to read and write data in interal and external slot. But the screen flashed every 3-4 second when I put it into internal slot and charge it.

Then I bought another one scandisk Utrl 2 4Gb clas 4 for the internal slot.It is no problem for now.

It said ,
Non-volatile solid-state; no moving parts maximizes battery power; data is not lost when power is turned off
Low battery consumption to maximize battery life in small portable devices
Operating shock rating of 2000Gs, equivalent to a 10' drop to the floor