Is there any risk in moving the contents of an internal hard drive TEMPORARILY to an external hard drive?

I have a 250 GB internal Hard Drive with 92% free space with Windows XP as the operating system. I know I cannot install a second operating system (Vista) on the same drive as it will simply perform an upgrade (replace Windows XP). The Vista will not install to my external hard drive (160 GB divided between two partitions). What I am thinking of doing is transferring the entire contents (approx. 11 GB) of my internal hard drive over to my external hard drive to enable me to install Vista on my internal hard drive. Can this work? Will there be any problems transferring the original contents of the internal hard drive back from the external hard drive? Let me break it down (1) transfer contents of internal hard drive to external hard drive (2) internal hard drive is now empty and can presumably be partitioned (3) Vista is installed on the now-empty internal hard drive which has now been partitioned (4) the original contents of the internal hard drive to be transferred back