I just found that my SD internal storage get very small every day, I did not install lots of thing, but now it is only 1.5G.
So you need to check yours as well on your computer to see is it happen to you. It is very serious problem which will cause SD care data lost in the end.
This is how you can check.
1 Connect your One X USB to PC. Use USB storage mode.
2 In file manager click on your X'sd card name and mouse right click choose properties, it show how much data used and how much left. Write done the number.
3 Click you SD card icon and pop up another screen show all folders and data in SD card, use Ctrl+A to select them all, then mouse right click and choose properties to show the calculation for how much all those files storage.
The used space for step 2 and step 3 shall be almost same if it is correct. But on my One x it has 13G difference. Which means the 13G storage is nowhere on the SD card but system think they are. So you will loose those space.
Do your check and see how many of you has this problem.