Deleting Volume ?

Del 80 said:

.....Yes, it's still there in Diskmanagement, but the format option is greyed out !!

Pentagon said:

@Del 80
Then open Command Prompt and run as Administrator
C:\WINDOWS\system32>diskpart
list disk
sel disk # (replace # by the disk number containing the (C:) (0 or 1 ?)
list par
sel par # (replace # last partition number
delete partition override
create partition primary
format quick fs=ntfs label="Data"
assign letter=D

copy&paste the commands and you are ready

Have you noticed that the D: partition is Active? This means that it has the boot files.
It is greyed to prevent deleting it. If you delete the D: partition the computer won't boot.
You must first make C: partition active and it may require to load the boot files to it.

@Del 80
What do you have on D: partition? Is it another Windows?
Is quite simple to make the changes so you can delete the D: partition.
Do you want me to guide you?

Find out if you have a C:\Boot folder. It is a OS and hidden folder.
Explorer options, View tab, must be set to show Operating system and hidden files and folders.