[JSLTIGER]

Switching from a 32-bit OS to a 64-bit OS is usually not a big deal, because all of the 32-bit programs will run normally. Unless you're using ~20 year old software that is 16-bit (b/c 16-bit is no longer supported by Windows x64), you ought to be fine. That being said, generally the only reason not to use a 64-bit OS these days is because you have an older computer with less than 4GB of RAM. If you have less than that, there is no point to using 64-bit software, but a 64-bit OS will let you use all of your RAM if you have 4GB or more.

As to your above question, you should not notice any difference in the functioning of your external HDD between 32-bit and 64-bit. Accessing your files should be as normal. Just remember that programs that you might have installed will not work because the registry will be written over. You also do not need to install Win x64 to the external HDD unless you want to boot from it, which I wouldn't recommend, because USB 2.0 is slow.