When you boot from a USB installation media (Windows 11/10) and select "Custom: Install Windows only (advanced)," you are taken to a screen showing a list of partitions.

If this is toggled to "Yes," Windows will wipe every secondary hard drive and SSD connected to the machine. How to Ensure Other Drives are Safe

Remove all USB thumb drives, SD cards, and external HDDs.

When you initiate a clean install, the installation process typically involves the following steps:

If your "C:" and "D:" drives are actually just two partitions on the same physical disk , deleting the entire disk volume to create a new partition will wipe both.

This is where the "exclusive" wiping happens. You will see a list of drives and partitions.