You may be unable to click "Next" in the Extend Volume Wizard in Windows Vista or Windows Server 2008

Source: Microsoft Support

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When you try to extend a volume using Disk Management in Windows Vista or Windows Server 2008, you may be unable to click Next in the Extend Volume Wizard.

The default size in Select the amount of space in MB is larger than the available space on the disk.

Change the value in Select the amount of space in MB to be less than or equal to the value listed in Maximum available space in MB.

You will then be able to proceed with the Extend Volume Wizard.

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