Typically when creating a new vm in virtualbox, using the new virtual machine wizard, you will eventually be asked what to use as your vm's bootable hard disk. Usually, this is a file, and that is what I suspect this is.
So, when you're installing fedora on the vm, the hd it will be referring to is the one assigned to the vm as the primary hard drive. (Through the new vm wizard, this would be the file you picked/created to be the primary hd.) There could be additional hds, but by the message you provided, it sounds like it is referring to the primary (virtual) hd. The first hd fedora would find is assigned to sda.
An easy way to tell is by looking at the main virtualbox window. If you select your current vm you are using for fedora, it will show the details of the vm on the right-hand side. It shows you what virtual hard drives are being used. If the vm is not running, you can select Hard Disks in that section, and it will popup a window with info relating to what hard drives are assigned to the vm, and where they're located at.
In my currently-installed version of virtualbox (1.5.6), choosing a real hd from the interface (ex. your host's hd) does not appear to be an option.