i had the same problem and abrixen's fix worked for me too:
24" imac 9,1 won't boot from any ssd if connected to the hdd bay cable
the screen stays white if the ssd is connected to the main sata port, where the hdd used to be.
no apple logo, no question mark at boot.
no option boot menu possible even if another bootable drive is connected to usb.
no option boot menu even if a bootable cd or dvd is inserted
just like @abrixen i took a standard red pc sata cable with a short straight plug on both sides, to be sure the bezel will close if it is plugged to the mainboard
reason for a short straight plug instead of an angled sata cable was, that any angled sata cable i could find had the angle pointing in the opposite direction, so if connected to the board it would point towards the wifi module
(a supermicro sata server cable was the one with the shortest straight plug i could find. almost as flat as the angled plugs as you can see comparing the original sata power cable to the supermicro sata data cable in the picture below)
to save time i didn't bother to route the sata cable below the main board, as it was so thin, that it fittet between the cooler brackets and the display and the temperature seemed to be the same to less compared to the heatsink side below the board, so if it is routed as on the picture the imac display will fit cleanly with no more pressure from below as the pressure the sponge spacers apply without an additional sata cable.
@abrixen: many thanks for sharing your fix with a picture.
without your pic i wouldn't even have tried using a standard sata cable because i'd not even have expected the connectors to be standard sata on BOTH sides.