I'm a PC user, so I'm speculating here but 10 GB free on your hard drive is problematic as IClone also creates temp files for crash recovery. So it may be that iClone takes that into account when looking at space requirements.
The only way to change the content storage location (apart from editing registry settings) is to uninstall and then reinstall while specifying a new location for Reallusion content, which can then be your external drive.
I don't know how bootcamp does things, but I assume that you can assign a drive letter (a typical windows thing) to the external storage?
You don't need to reinstall content. Once your new drive is in place you can just copy it over from the old location. The exceptions are the Resource pack and any Bonus packs that come with iClone. They need to be reinstalled.