Bootcamp question: creating bootable Windows drive from an external HD partition?

Hi,

So I just got a new rMBP, which obviously doesn't have an optical drive. In the past, when I've installed Windows with bootcamp I've used the DVD method, but this time I'll need to use a USB drive. All the USB thumb drives I have are too small for this (4gb), so the only other option I could think of (other than going a buying a larger one tomorrow) would be to use one of my external hard drives as the bootable Windows drive.

I understand that the drive you use to boot from when installing Windows will be reformatted and erased. I have an external HD with two partitions -- one partition is important stuff (backed up, but important nonetheless) and the other is just a beta of Yosemite I tested a while back (which I don't need at all). If I erased the data on the partition that I DON'T need and used it as the bootable Windows drive, would that in turn also erase the important stuff on the other partition, or would the "important" partition be safe? I've tried googling this for a while but can't seem to find an answer. Does anyone know?

 

Yeah, it's just more the hassle of waiting until sometime tomorrow to get things underway. Would rather get it done now (impatient, I know). Anyway, thanks for the response.