In general, there is no reason to have more than one partition on an external hard drive. There are some specific cases where it might be desirable but you would know if you had a requirement like that.
Modern disk drive construction more or less requires drives to have at least one partition. Whether the drive is internal or external is immaterial.
You can certainly move Windows to an external hard drive but Windows will not boot directly from an external drive. If you are running Windows in Parallels (See links below) you can have Parallels installed on the Mac's drive and then have your Windows virtual machine on the external drive.
The system partition(a partition where the operating system is installed) is the active partition of the Hard Drive
system partition
System partition
There are loads of partition management programs on the 'net. Paragon is a good one - it lets you set the partition sizes - and shows you any unallocated space.
You should not have to partition an external hard drive, especially with that much space. if you must partition the drive, you can download a partition manager like PartitionGuru online, and most of these programs are for free. They can partition hard drive, external hard drive and USB flash drive. But you have to read tutorials first before partitioning anything. If you have important files on the drive, you should backup files first.
Most of the time the boot partition and the system partition are the same partition on the drive C.
The area on the hard drive that contains a map to all the partition on the drive is called the partition table. That is what partition utilities edit when you add, delete, convert, or resize a partition.
extended partition
No. A computer can have any number of drives. A: and B: are reserved for floppy drives, and C: is reserved for the first hard drive partition. The rest of the letters can be assigned to anything. D is usually given to either a second partition on the hard drive, or a CD/DVD drive. However, some computers, such as netbooks, may only have one partition on the drive and not have a CD or DVD drive. If you were to plug in a drive, such as a USB Flash drive or external hard drive, then it would become drive D:.
Most of the time the boot partition and the system partition are the same partition on the drive C.