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Right click the hard drive in which you wish to create a partition. If there is unused space, you will be prompted to choose New Simple Volume. If not, then the option you will want is Shrink Volume.
If you have chosen the Shrink Volume option, you will be prompted to enter the size you will want to be able to shrink. By default, it will give you the total amount of available space you can resize. However, if you want less, you can enter it here as well. Note that it will only give you a figure in megabytes as opposed to gigabytes. 1000MB equals 1GB, so calculate accordingly. Afterwards, hit Shrink, and your new partition is created. If prompted to format the unused space, you may do so if desired.
If you have chosen New Simple Volume, proceed to Step 3.