I disconnected all peripheral power and USB power but no change. The only connections from the power supply are to the motherboard and cpu.
Do I also need to disconnect the SATA connectors?
Should not be necessary if you disconnected the SATA power leads, and is unlikely to help, but couldn't hurt.
Do I need to provide power to the graphics card for the motherboard to boot?
Unless you've got integrated graphics you probably need a GPU to POST. But you should at least be able to get the CPU fan to spin up.
It is possible that the GPU could be shorting something out even with its power lead pulled, since it can also draw power from the PCIe slot. I would actually recommend pulling the GPU and switching to IGP (if you have one), or installing a spare older GPU to rule out the possibility that the culprit is the GPU.