Hi,
Thank you for posting the query in Microsoft community forums!
In fact, "Extend Volume" option only available when there is contiguous unallocated space behind the volume which you want to extend. As here is no unallocated space after the C partition drive, so extend volume greyed out.
You need to have an “unallocated disk space” to the right of the Partition\Volume you wish to extend on the same drive. Only when “unallocated disk space” is available “extend” option is highlighted or available.
Refer to the links:
Extend a Basic Volume
http://technet.microsoft.com/en-us/library/cc771473.aspx
http://technet.microsoft.com/en-us/library/cc770938(WS.10).aspx
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