Take a look at the partitions in disk manager. Some drives come with a tiny partition on it with various manufacturer's tools. This picture is just a drive with a partition taking up 2 TB.
Then we’ll need to run diskpart.
Type ‘diskpart’ in the command line and in the Diskpart prompt type ‘List Disk’.
You’ll see that this 3TB drive is split into two volumes.
Select the disk you are trying to clean. It is
VERY important to select the right disk, because Diskpart is very powerful and can wipe your windows partition right out from under you if you select it instead. *
Now that we have the right disk on the chopping block, clean it by typing ‘clean’.
You’ll notice that it didn’t ask me if I was sure. Now this disk is totally clear and ready to be made into a NAS drive. It now looks like this in windows :
And that's it, should be ready to go.
* Actually, diskpart won't clean a boot or system partition. It will still mercilessly delete all your data on a storage hard drive.