Use power supply w/o mobo?

Suggestion 1:

Switch to an AT style supply instead of an ATX style supply.

With an ATX supply, the power switch is a momentary contact push button that signals circuitry on the motherboard to turn the power supply on to normal mode.
The motherboard then signals the power supply that all is good, and keep running.

On an AT style power supply, the power switch actually switches the AC line power to the power supply. Among other things, this provides a hard power off condition.

Suggestion 2:
Try a lower wattage ATX supply.
Even though you are running 9 drives, you may not need more than a 350 watt power supply.
If you have a large power supply, the static load of the drives may not be enough to provide the minimum load for the power supply to run.

Option III:
Do you have basic electronics knowledge, mechanical skills, and tools?
You need a circuit to fool the power supply into seeing a motherboard.

One way to do this is with an ATX power supply tester. Simply remove the load resistors so that the tester does not overheat.

Or, you need a circuit that properly connects the power on and power okay lines from / to the power supply.
Check out the information at:
http://www.hardwarebook.net/connector/power/atxpower.html

Chas