To make another partition on an existing drive, right click on My Computer and select 'Manage'. In the 'Computer Management' window, under 'Storage', click 'Disk Management'. Given your current partition doesn't take up the entire hard drive, you can right click on the 'Unallocated space' part and choose 'Format', or something like that. If you don't see 'Unallocated space', your current partition most likely takes up the entire hard drive, therefore you cannot split it unless you reformat your disk and make a new partition that takes up about half, then make another partition on the other half, then assign it drive letters by right clicking on it and choosing 'Change Drive Letter and Paths'.
If you want more space, though, I would suggest you just buy another hard drive
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