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There is no need to download any 3rd party software, you can fix this in
windows using disk management. To get to disk management click the start/
windows button in the bottom left, right click on "computer", left click on
manage, under "storage" click "disk management".
The disk needs to be empty so delete all volumes so the entire disk is
Unallocated, then simply right click on the disk number (ie disk 1, disk 2,
disk 3, etc) and click on "convert to GPT disk". Takes two seconds to do and
now you have your extra 746.39GB.
Keep in mind this is only supported in Windows Vista and higher. If it doesn
't work, you may need to enable UEFI in your BIOS, granted your motherboard
supports it.