If this is a portable or inside a console for your tv the pi has too large of a 5v current requirement to run directly off the motherboard. In the case of for your tv a step down from the power adapter input. In the case of a portable and you're running a 2s battery setup it's the same thing. Advanced trim PS2 requires more finer building of your power source but it will be a similar fashion you'll just have to make sure your 5v source can handle the current.
My answer still explains it, stepdown voltage from the power input on the PS2 slim. There is no 5v point on the PS2 that can handle the current for a raspberry pi
You'll need to source 5v step down with a good enough power draw for the pi3 and usb drive.
I missed the portable portion initially when I read 8.5v adjusted my answer.