iMac won't detect new HDD or external drive

Hi, i've recently bought an iMac 27" from 2009. I bought the iMac without a HDD and optical drive. The person who sold it to me said that there's nothing wrong with it (other than it didn't have a HDD or optical drive). So i installed OSX El Capitan on a seagate HDD (which works), and formattet it to GUID and the mac journaled thing and used a SATA-cable with thermal sensor. But, when i start up the iMac, it shows the question mark folder. So i tried to hold down option, when i boot the iMac, but there's no HDD to see... not even my external HDD with OSX El Capitan installed and with the correct format, only my mouse appeared. Then i tried to reset NVRAM, which didn't help either.. it wouldn't even react to me pressing down option, command, p and r or even option, shift, p and r. i took a look at the logicboard to see if the cables wheren't plugged in, but they were (i also made sure all the new cables for the HDD were properly installed). And i can't open disk utility to show all drives, because there's no MacOS on it. by the way, i can't access recovery mode or anything.

So is it the missing optical drive, which makes it show the question mark or is it a deeper and more advanced problem?