Only mmc2/mtube was read-only.
I had mmc2/map that was consuming most of the space.

Here is what I did:
I copied all data I wanted to a laptop via USB.
I reformatted mmc2 using File Manager. It has been working great since then.

I love this application. I still think the volume setting stay at the last value when we move to the next video, especially when using headphones.