You can partition any HDD with programs like ,
Partition Magic
Acronis
Paragon
Parted Magic
You will have to search for walkthroughs and manuals for information on how to use the programs. Some are free to use.
If you decide to install Linux on a HDD then you usually have the option to use GRUB as a BOOT LOADER, this will find all os's installed and you will get a choice at boot up as to which os you want to run.
If you do not run a Linux os then you will probably need a seprate BOOT LOADER to use windows os's on a external HDD. Acronis has such a facility but am not sure if the True Image sreies has this facillity.
You will need to ask more specific questions about your intentions and the tools you will use if you go this route. Also reading up on partitioning and installing os's will help as this is not an easy task. It is not as complicated as some folk make out. It is just that there are certain things that need to be done in a certain order for it to work smoothly.
Most important thing is to have a backup of your present system in case of cock ups.