keylight
Veteran
I am familiar with the Transformer. But unless I am mistaken, it only has a microSD slot, with the full sized SD on the optional keyboard dock. I love the idea of the prime because it can lose the keyboard for easy travelling, and then add it when you want/need a more laptop-like device, but for what the OP is looking for it may be silly. I mean, he could just opt for an ultrabook or macbook air (arguably about the same size as a transformer w/ dock), but I think the idea is to travel light (ie. just a small cheap tablet).
It has a MicroSD slot on the tablet, and full SD on the keyboard. I use it with the keyboard for transfer. I always use the keyboard (using it right now for this reply!) and only disconnect and use it as a tablet if I'm reading or watching stuff.
But.... Asus makes 2 very cool little adapters, each less than $20. They both plug into the power port on the bottom of the tablet, so you can use only one at a time, which means transfering in two steps.
First, you use this, which will let you read your SD cards. Put a microSD in the tablet and you can copy all files off of the SD card to the microSD card:

Once that's done, unplug it and plug in this one:

This will let you plug in your USB HD and transfer the files from microSD to the HD.
That's it. Although I wouldn't bother with this stuff and just go the keyboard route. I love that the keyboard gives an extra 6-8 hours of battery life, and the extra 1.2lbs is worth it.
After using the TF700, I'm pretty sure I'll never buy an iPad. It's just such an awesome device.
EDIT: Although since the OP is backpacking, I could see not wanting an extra 1.2lbs, so these two little adapters may be the better option.