You can not use GUI interface to format a disk with windows installed. DiskPart is the command line version of the Disk Management graphical interface. It allows you to view, create, delete, format, and manage any connected drives on the system.

Open a command prompt, type in:
diskpart, and press Enter
list disk — viewing the available attached drives.
select disk 2 — if disk 2 is the one you want to delete.
list partition
select partition 1
delete partition
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clean –when you’re in the disk you want to work on, it will wipe the entire disk
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