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#11
You can expand the C: drive back to fill the empty space, but for reasons you have seen I recommend using MiniTool Partition Wizard for that. You can also create an NTFS partition in the empty space you created and assign it a drive letter and you will have storage space that way.
The way that we recommend "trying" Windows 10 without violating the EULA requires an external hard drive or network storage location. We recommend you create an image of your existing hard drive using a program such as Macrium Reflect Free. The image will be about 80-90% in size of the used space on your hard drive. The free space does not get backed up because there is nothing there - unless you tell it to back up sector by sector. Then you upgrade your existing installation to Windows 10. If you don't like it, then you restore the image you made, erasing Windows 10 in the process of restoring.
If you want to see your Windows product keys, run showkeyplus from this forum:
Showkey - Windows 10 Forums
Here is MiniTool Partition Wizard:
Best Free Partition Manager for Windows | MiniTool Partition Free
Here is Macrium Reflect Free:
Macrium Reflect Free