Reddit reviews Apricorn USB Power Adapter Y Cable AUSB-Y USB (Black/Grey)

If you can get your hands on a laptop hard drive or ssd that would solve your problem. You can also get a Y usb cable to provide additional power (http://www.amazon.com/Apricorn-Power-Adapter-Cable-AUSB-Y/dp/B000JIOHDE) with a usb power supply (http://www.amazon.com/Powergen-2-4-Amp-Charger-Designed-Android/dp/B0073FE1F0/ref=sr_1_2?s=electronics&ie=UTF8&qid=1417877055&sr=1-2&keywords=usb+power+adapter)

With this sort of y cable you can also use a battery pack as USB power. That preserves the portability.

If you want to maintain the connection to the audio system you need another charger and something like this (I don't have one, just showing an example):

http://www.amazon.com/gp/aw/d/B000JIOHDE/ref=pd_aw_sims_2?pi=SL500_SS115&simLd=1

That will let you pull power from both while using the data connection with the car. If you don't care about the data connection then just get a separate more powerful car charger and use that. I'd look for something with at least 1 amp of output. I have one of these that does well:

http://www.amazon.com/gp/aw/d/B0056VNVV8/ref=mp_s_a_1_1?qid=1420915220&sr=8-1&pi=AC_SY200_QL40

Edit: you can use a charger like that in either situation. Just wanted to clarify.

I want to create a USB powered 4 port hub setup for the shield. I'm curious as to the safety of having 2 separate USB power sources connected at once.

Basically it goes from shield to OTG Y cable powered by USB to a USB Y cable powered by another USB then to 4 port hub.
The OTG & USB Y cables will each be powered by a standard 2.4 amp USB wall charger. Maybe car charger too
The OTG Y cable alone is able to power the shield tablet as well as a hub but higher power devices make the shield charge slow or not at all, possibly sending power back to the USB device.

Will this setup allow the hub to pull more than 2 amps assuming the USB Y power source can output?

The OTG Y cable is on amazon, "Apricorn USB power adapter Y cable AUSB-Y USB"
http://www.amazon.com/Apricorn-Power-Adapter-Cable-AUSB-Y/dp/B000JIOHDE

Will this potentially fry my shield?

Yes, you want a Y cable. They use them for hard drives all the time. One connector is power and data and the other is just power, and usually is red. Hook the data cable to your stereo and the power cable to a usb car charger. Done. I've been meaning to do it in my car but my list of things to do is never done. Example. You can find them with whatever connector you want on the end as well if you want one less cable.

The Toshiba drive (and the other external drives) do they have their own power source or are they drawing power from the laptop's USB?

When they're plugged in, do their LED's come on (and get steady) and can you hear/feel the HDD spinning?

If it's an older laptop, it might not be giving enough power to the drive over USB, the ports might be worn out. Combine that with slightly older external drives that also have warnout USB ports and that might be the issue.

(Not enough power can sometimes be heard if the HDD's are clicking softly)

A different (new) short USB cable might help, a USB Y cable could too since it uses 2 USB ports, one is dedicated to powering the external device.

Maybe not this exact product, but something like it?

http://www.amazon.com/Apricorn-Power-Adapter-Cable-AUSB-Y/dp/B000JIOHDE/ref=cm_cr_pr_product_top

Can you inject DC power into the chain somewhere? I may have misread your description, but if I did, maybe this gets you thinking about a different option.

You can try one of these:
http://www.amazon.com/Apricorn-Power-Adapter-Cable-AUSB-Y/dp/B000JIOHDE

But no, there's no easy way to increase the voltage on a USB port.