My fiancé has a e-book PRS-300, and it just get stuck with a warning "Invalid Format!" and a big exclamation mark inside a triangle in the middle of the screen, and I just can't access its options. So when we press "ENTER" the screen clears and a message appears with a title "Format Internal Memory" - Press MARK to format internal memory.
Then when we press it, a message appears below "Formatting Internal Memory..." but a phew seconds later the big exclamation mark inside the triangle appears again, and below the message "Invalid Format!".