To install Windows on a new HDD without affecting the existing version on an old drive, the safest approach is to first remove the old drive before installing Windows on the new one. After the installation, the old drive can be reconnected as a secondary drive, allowing for data transfer. It is crucial to avoid formatting the new drive or installing an operating system on it while the old drive is connected, as this could lead to accidental data loss. The recommended steps include: removing the old drive, installing Windows on the new drive, reconnecting the old drive as a secondary drive, and then transferring data from the old drive to the new one. Keeping the old drive as a backup is also advisable.