For this job I use Easeus Partition Master Pro. Some folks here use Gparted, but I have no experience with it.
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Good luck.
After installing Windows 8 on my main partition C, it is getting nearly full. So, I just have tried to merge some free space from Partition F in Disk Management. But, that doesn't work. Some people tell me that is because the free space is not adjacent to my C partition. I don't know whether it is right. I simply want to extend my C partition successfully. Can you help me? Thanks advance!

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