External USB hard drives, thumb drives, and Network Attached Storage (NAS) units are entirely safe from the automated installation process. The installer will never touch external media unless you manually command it to do so. The Risks: When a Clean Install Can Wipe Other Drives
There are two specific scenarios where "Clean Install" effectively wipes "all drives," but they require active user intervention: does clean install wipe all drives exclusive
If you want to be 100% certain that your secondary drives remain untouched, follow the rule: External USB hard drives, thumb drives, and Network