When you get your Graphical login you are on run level 5. The system already passed by the run level 3. It is in this level you see the boot establishing the network. From this level it goes to level 5 and starts X, the graphical environment.
The most important notion is in *nix/Linux all you have are directories and files. You don't have c:, d:, etc. Your root partition is mounted in the root (/) directory and all the other are mounted in sub-directories of root. So, /home is a sub-directory of root and may be pointing to an area of the root disk or point to another partition in the same or another disk.
To mount any partition in the /home directory all you need to enter is
mount /dev/xyz /home
Of course this paritition must be initialized and known to the kernel. "/dev/xyz"in the example must be replaced by the partition you want to mount. You can even mount files as pseudo disks, once you create a file of a needed size and format it. Note, if you have some files in a directory and use it to mount another partition, these files will be hidden while the partition is mounted.