Detailed information below. For simple steps, skip ahead to step 1.
I wanted something like a file/media server setup in the family car. The problem is that I don't want to put a computer that would have to boot up or shutdown (I'll be there in a minute...I'm waiting for the computer to shut down). There are other scenarios that could improve that but I like to go as minimal as possible in terms of hardware. Hopefully less hardware=less problems. So why not get a wireless router with a usb slot for NAS? Sounds reasonable enough but what about the price? Prices have dropped to a reasonable amount for these types of routers. I was in a store recently and I come across two Belkin N600 HD (there are two types of these. I have the older generation...with two usb ports) for $10 each (used/returns). Here is a link to it on Amazon: http://www.amazon.com/Belkin-Wireless-Router-Older-Generation/dp/B0047T7AIU/ref=sr_1_5?s=electronics&ie=UTF8&qid=1389548500&sr=1-5&keywords=n600+hd+f7d8301 Another option for an affordable device may be a TP-Link TL-WR703N http://www.amazon.com/TP-LINK-TL-WR703N-Portable-802-11n-Wireless/dp/B0083Z54P0/ref=sr_1_2?ie=UTF8&qid=1391183761&sr=8-2&keywords=TP-Link+TL-WR703N or the RAVPower hotspot http://www.amazon.com/RAVPower%C2%AE-Wireless-External-Streaming-RP-WD01/dp/B00AQUMZRA/ref=sr_1_1?ie=UTF8&qid=1391183660&sr=8-1&keywords=rav+wifi but I haven't tested either of these.
So what I used was one wireless router with a usb port (usb 2.0), a jump drive, and a DC to AC inverter. I want to eventually get rid of the inverter when I get a 12v 2A cig. plug power adapter. You basically setup the router like you normally would any other router. You can plug it in and start using it without changing any settings. I changed the security settings on mine. I wouldn't want anyone tapping into my wifi network while traveling down the interstate. I loaded some files onto my jump drive and then attached it to the router. The LED above the usb port lights up when it is ready. On a windows computer the network share will automatically show up in your network as ROUTER. So now the router is ready to be placed in the car for future use. I have Kindle Fires, an Android phone, and an Iphone. They all use different software to be able to get to the media. I use Avia (Chromecast) on the Kindles and Media: Connect on the Iphone. There's a lot of other apps out there that will work but that is what I already had installed on my devices. While in the car I have had three Kindles playing the same video separately without any problems. The router I used only supports self-powered portable USB hard drives or thumb drives so be wary when you buy yours.
Gather the basic items needed:
1. Wireless router with a usb port.
2. Usb flash drive or other usb hard drive. I will be using a flash drive in this instructable.
3. Power inverter.
You will also need a computer to load your drive with media.
Plug up your router and configure it the way you want. I will not go into detail because you may not have the same kind of router that I am using and you really don't have to change anything in the settings for this to work. I would suggest changing the default security settings because you just never know.
Plug your jump drive into your computer and transfer all the media(music, movies, pics, etc.) that you want. Once that is done, unplug it from the computer and plug it into the router. My router has an LED on the back. It will light up once the drive is ready to be shared.
Install your apps on your devices.
Android devices: I chose Avia Media Player (Chromecast) to browse the networked jump drive but there may be a better app out there. Please list your favorites in the comment section.
Apple devices: I chose Media: Connect.
Both apps automatically detect the router drive. Make note that the router/server may not be available if a drive is not plugged into the router.
Finally you take the router and place it in your vehicle and plug it into the inverter. The inverter can be eliminated if you have a power adapter that will plug into the cig. plug. Connect your devices to the wifi network and enjoy all your media.
Do not leave this on overnight. The power draw is enough to drain the battery. It had to be said.
If you use a self powered hard drive I'm not sure if you can safely unplug it or turn off the power without risk of damaging the drive. I did come across this interesting article about removing usb devices from the computer. http://howto.cnet.com/8301-11310_39-57425717-285/quickly-remove-usb-devices-without-using-safe-removal/
Instead of disconnecting the drive to add media you can simply connect to the router in your vehicle from within your home and then transfer them wirelessly. Of course you would have to be within range and have the router powered on (there's always a downside).