This repartition is actually pretty easy.
Please note that it's normal to install Windows first, and Ubuntu second, and both should be installed in the same fashion... either BIOS mode or UEFI mode... with a new GPT partition table (which will wipe the disk) if your computer is in UEFI mode. It's best to make these decisions now, rather than trying to change it all later.
Make sure that you have a good backup of your important Ubuntu files, as this procedure can corrupt or loose data.
Keep these things in mind:
always start the entire procedure with issuing a swapoff on any mounted swap partitions, and end the entire procedure with issuing a swapon on that same swap partition
a move is done by pointing the mouse pointer at the center of a partition and dragging it left/right with the hand cursor
a resize is done by dragging the left/right side of a partition to the left/right with the directional arrow cursor
if any partition can't be moved/resized graphically, you may have to manually enter the specific required numeric data (don't do this unless I instruct you to)
you begin any move/resize by right-clicking on the partition in the lower part of the main window, and selecting the desired action from the popup menu, then finishing that action in the new move/resize window
Do the following...
Note: if the procedure doesn't work exactly as I outline, STOP immediately and DO NOT continue.
- boot the a Ubuntu Live DVD/USB
- start
gparted
- right-click on /dev/sda2 in the lower pane and select resize
- resize the right side of /dev/sda2 and reduce it by 400G
- right-click on the newly created unallocated space and create two new 200G partitions (EXT4 if it's for Ubuntu use, or NTFS to share with Windows, or NTFS if it's to become a Windows installation at a later date.)
- if everything looks correct, click the Apply icon
