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You must open Disk Management and document the Disk Number of the drive containing the GPT Protective partition, as you will need this information later in the procedure.
Determine the disknumber assigned to the GPT-protected drive.
Right-click on (My) Computer.
Choose Manage.
Select Disk Management (listed under Storage).
Look for the drive that is identified as GPT and note the Disk number (such as Disk 1).
Open a Command Window. From the command prompt, type diskpart and press Enter. The diskpart prompt will open.
From the diskpart prompt, type list disk and press Enter. A list of disks will appear in a text format. You will return to the diskpart prompt.
From the diskpart prompt, type select disk disknumber (for instance, if the disk containing the GPT Protective partion is Disk 2, you would type select disk 2)and press Enter. A message appears saying that the disk is selected. You will return to the diskpart prompt.
From the diskpart prompt, type clean and press Enter. At this point the drive's partition and signature a removed. You will return to the diskpart prompt.
From the diskpart prompt, type exit and press Enter. Type exit once more to close the Command Window. At this point, the internal and/or external drive can be re-initialized, partitioned and formatted.
Launch Disk Management, and to initialize the disk:
Use the Initialize and Convert Disk Wizard...
-OR-
Close the Wizard, right-click on the disk in question and select Initialize Disk from the drop-down menu.
Once the drive is initialized, continue using Disk Management to partition and format the drive.