How many of those devices are also powered through the USB. That is my problem, the usb drive is a portable drive powered through usb off the computer, with the ignition off, the car can't provide enough power to run both.
As far as the voltage, I think I can use the software for this device to make it output 9.5V (although I'm not 100% sure about that I haven't looked that much into it).
If I can't get 9.5V would 9V or 10V be close enough?
My OLD 2001 Mitsubishi Eclipse GT:
"The Project That Never Ended, until it did"
next project? subaru brz
I'm thinking that you didn't get the voltage on your converter correct, and that is what's causing the computer to flip back and forth from battery to AC with the hard drive plugged in...
I definitely would not settle for 9V or 10V. You NEED 9.5v!
If you are able to check that you have a proper 9.5V from your converter that is powering the Asus, and then you still get the issues with there not being enough juice for the hard drive, then I would try these things:
1. Try running a direct power line from the battery with a relay to switch it on and off with the ignition, instead of tapping another line that's already in the car... I know that some cars will reduce the power to their cigarette lighter once the car's engine is off (even if it is in the "on" or "accessory" position) so that it doesn't drain the battery... i.e. make sure that your 12v source is still a full 12v under load when the ignition is on but the engine is off.
2. If that doesn't change anything, try getting a separately powered USB hub, and power it with a separate DC-DC converter, probably will be 5V 1-2A. Then plug the hard drive into that. This way the hard drive will not be drawing any power from the Asus. Alternatively, you could just get an external hard drive that requires its own power source, and then match the voltage on that. Or, if your hard drive is the kind that has the pigtail dual-usb plug that plugs into two ports, so that it can use one for power, you could just cut that cable and supply 5v directly to the pigtail.
3. Try using a flash drive instead. Are the ECU tunes that you are putting on your car so large that they won't fit on a 8GB or 16GB flash drive? Flash drives will require significantly less power than a regular hard drive.