Okay. This is happening in a car.
I have an aftermarket stereo which has a USB port.
I also have a USB hub that requires power.
I've successfully stepped down the voltage from 12v to 5v using a converter.
The converter is wired to the RETAINED POWER wire on the radio harness and grounded on the ground wire of the radio harness (Retained power is hot when the ignition is on and remains hot for 10 minutes after its off OR someone opens the vehicles door while the ignition is off.)
The radio receives its power and ground using the same cables.
The radio ALSO receives CONSTANT POWER from the "BATT" cable which is ALWAYS HOT (Used to keep the time and date.

WHEN THE USB HUB ALONE IS CONNECTED TO THE CONVERTER, IT FUNCTIONS AS IT SHOULD, RECEIVES POWER WHEN IGNITION ON - DIES WHEN IGNITION OFF AFTER OPENING DOOR OR WAITING 10 MINUTES

WHEN THE RADIO ALONE IS CONNECTED TO THE POWER SOURCE IT FUNCTIONS AS IT SHOULD.

WHEN THE RADIO AND USB HUB ARE BOTH CONNECTED TO THE SOURCE THEY FUNCTION AS THEY SHOULD....
UNTIL
YOU CONNECT THE RADIO USB PORT TO THE USB HUB...
When this happens NEITHER DEVICE GOES OFF.
I removed the Inverters ground and both devices remained ON.
I concluded what must be happening is that CONSTANT POWER from the BATT cable is sending current through the radio (intended to keep the clock time) and traveling through the USB cable, into the HUB and back down POWER CABLE of the USB HUB into the RETAINED POWER cable and powering the head unit, making it as if it was never turned off... The issue is... when the radio is SUPPOSED to be off, power is backfeeding down my USB Hubs power cable and into the RETAINED ACCESSORY cable and keeping it on..
If I use a relay, it would have to be wired to the retained power cable to activate when power is triggered, the PROBLEM with this is.. like my radio, it would always receive power because power is flowing the wrong way.and the relay would never disengage.
If I used a diode, I will experience a voltage drop and I must keep it at 5 volts after the transformer...
If I use a MOSET on the USB hubs POWER CABLE, then the power coming up the USB cable from the radio which is received ALWAYS HOT battery power...will still power the USB hub because the MOFSET will short it to a ground when current is going the wrong way so the power will go from my radio, up my USB Cable, Into the USB Hub, OUT my power cable and into the MOFSET ground thus keeping my USB Hub ALWAYS HOT..which will drain my battery...

Are there any ideas or ways to attack this?

 

Draw a diagram of what you have.

 

Draw a diagram of what you have.

The Image with the green arrows shows as to what I BELIEVE the path of electricity is when the radio and USB hub are supposed to be off (key off / door open) - I'm not 100% sure on this.. but its seeming this way.

 

Don t use the retained power as a supply, instead use it to switch a relay.

Add a relay that is switched with retained power.
Add a fuse, preferably 2, and take bat, via the fuses, through normally open contacts on the relay to the converter and radio retained power input.
Your converter and radio retained power connections will now switch properly with the car circuit.

However there is still a back feed between BAT and the hub power rail via USB+, assuming that that was the problem in the first place, which seems likely.
If USB+ turns off when the retained power to the radio is off you are good however if that is not the case and USB+ is effectively at 5V whenever Bat is on, constantly, then you will need an additional contact on the relay to break the USB+ between the radio and the hub.

That said, since you don't need power from the radio USB port and the signal reference for the USB interface is 0V you can probably just sever the USB+ which would fix the problem without any additional bits.

There is a possibility that the hub is using the + line to detect active input but I doubt it, as if that were the case then simply connecting its 5V supply would be signaling the prescence of an input, given that USB+ and 5V in appear to be connected.

If it were me I would cut USB+, which will fix the back feed issue, and see if the interface still works.

If it doesn't then add a relay as described, which will definitely sort it out for you because the back feed will be interrupted and the retained power line, which switches the relay, will be isolated from the back feed, allowing it to turn off.

One last point...
The back feed must be 5V which, since it is causing the radio to turn on suggests that the retained input is only a signal as opposed to being a supply line that must be at 12V.
Given that the converter will stand some voltage drop on its input and retained power in on the radio only seems to need 5V you can probably just feed the retained power supply to the radio and hub supply via two diodes, isolating them from each other, without using relay contacts.

If USB+ is off when the retained power is off then you are good, if not, the radio will turn off but the hub will be on constantly and you will need to break the USB+ either perminantly, if that works, or with a relay switched by retained power, before the diodes.

Hope that helps,
Al