Obviously should the user have no need for any of the programs/data residing on the secondary HDD following, for example, a disk-cloning operation where the HDD was the source (boot) drive, then it's a simple matter (as Colif points out) to use Diskpart's "clean" command and then initialize, partition, and format the drive for further use.
But the following assumes the user is interested in deleting only the Windows folder/files and perhaps other folders/files on the secondary HDD that ordinarily the user cannot delete by the usual process of deleting a folder or file. Windows will generate a "Folder Access Denied" message that "You need permission to perform this action".
The Windows OS balks at a user's attempt to delete the Windows folder, even when that folder resides on a secondary drive in the system and not on the boot drive. While there are ways for the user to take control of that Windows folder and delete its enormous number of files, the process is quite complicated and generally very frustrating for the user.
As a general proposition I'm loathe to use a third-party (non-Windows-integrated) program to achieve an objective involving the Windows OS. I do so only when I've concluded that utilizing such a program is the only reasonable alternative in achieving the objective and that I'm reasonably assured whatever registry changes are made by the program won't play havoc with my current system and (hopefully) won't adversely affect my system in the future.
I mention this because I'm recommending a third-party program for your consideration in order to delete the Windows 10 OS from your secondary HDD.
A number of years ago someone came up with a simple-to-use program that achieves the objective of deleting the Windows folder (again, we're referring to the Windows OS residing on a SECONDARY drive in the system, NOT the boot drive). It's the "Take Ownership" program - see
https://www.tenforums.com/tutorials/3841-take-ownership-add-context-menu-windows-10-a.html
for a copy of the program. The website's description of the program and its use is quite clear.
Basically the registry change made in the OS will add a "Take Ownership" menu item that appears when you right-click on the Windows folder. After clicking that menu item the program will "mark" the nearly 200,000 files in the Windows folders in a manner that allows the user to "take ownership" of them. In so doing the user can delete all the files/folders in the Windows folder. It's pretty much an automatic process although during this process the user will be asked to confirm the deleting of this or that file. The process is not terribly quick but it's not unduly lengthy and there's very little intervention needed from the user during the process.
We've been using this "Take Ownership" program for a number of years going back to Windows 7 and have not experienced any problems or untoward events involving this program. But as we all know when it comes to PCs, #!)* happens!. That why we ALWAYS recommend that the user FIRST CLONE the contents of the HDD to another drive BEFORE undertaking a major change (such as this one) in the configuration of the HDD involved when there's data on the drive the user desires to retain aside from the data the user is deleting by this "Take Ownership" program.
Capiche?