My experience is that a bootable flash drive erases all the info on the drive while the boot is being installed. The only way to get around this is to partition it ahead of time. After it is made bootable, however, you can add other files to areas that are not part of the boot areas, and still boot up with it.
A friend bought a 250 gig solid state hard drive for $140. Yet you could go to Walmart and get two 128 gig flash drives for about 2/3s of the single hard drive. My question is, is it possible to run a computer off one of those hard drives made bootable, and simply use the other flash for storage... while having no hard drive at all? This would be cheaper, but not as fast as the solid state hd.
If you did this, you could take your info with you wherever you went, without leaving any of it on your computer while you were gone.