I did this on a recent trip using a tablet (Samsung Galaxy Note 2012) connected to a powered USB Hub/Card Reader.
This is the setup:
I used the hard drive because of limited space on-board the tablet and the hub needed to be powered because the tablet doesn't have sufficient power to drive it. Power was provided by the standard AC adapter for the tablet.
The hub/card reader was a cheap one I got on Ebay.
It all worked well - I just connected it up at my accommodation in the evening and let the file transfer occur while I went and did something else - such as had a glass of wine while watching the sunset. ;-)
I wasn't too concerned about viewing the raws (I was shooting mainly jpgs anyway) but there are apps that will do that on Android and I assume for iPad as well.
In retrospect I could probably have used large capacity USB thumb drives rather than the hard drive and they probably wouldn't have need the power.
Cheers
Brian