It's automatic and the makers can change this. If you need the details, ONLY the maker can answer.
For now I can write that I liked the first hybrid so much that I bought a second to replace a good drive!
How is it determined to which hard drive will the information be written to if the computer has both a SSD and a normal Hard Disk Drive? Can someone provide an explanation or a link to a good explanation. And if the SSD is for the OS does that mean that nothing but the OS goes on the SSD?