I wanted to update my GL702VM with an additional SSD besides the M.2-SSD in the M.2 slot (which keeps boot partition and system).
I recognised, that the laptop wouldn't boot at all without an HDD at the SATA Port nor with a SSD installed to it.
There is no POST, ROG or other message on the display, no starting fans, no sign of life - just a light on the Power LED.
I've updated the BIOS and checked the SSD in another PC, which works just fine. I switched the pre-installed HDD with another one and the laptop starts normally. Removing the HDD results again in no boot at all.
So to summarize my problem:
M.2-SSD + any 2.5" HDD => bootable
M.2-SSD + 2.5" SSD => not bootable
M.2-SSD alone => not bootable
Again, this has nothing to do with the boot partition, the laptop doesn't get to the state where this is checked via bios.