Prevent guest from reading an external hard drive
Why not just encrypt the drive?

https://www.howtogeek.com/184675/how-to-encrypt-your-macs-system-drive-removable-devices-and-individual-files/amp/

Encrypt Removable Devices

Mac OS X allows you to encrypt an entire drive, like a USB drive or external hard drive. The contents of the drive will be encrypted with a passphrase you choose and no one will be able to access them without that passphrase. It functions like BitLocker To Go on Enterprise editions of Windows, but it’s available to all Mac users.

To encrypt a drive, simply open the Finder and connect the drive to your Mac. Ctrl+click or right-click the drive in the Finder sidebar and select the Encrypt “Drive Name” option.

The disk will be encrypted once you enter your password of choice — be sure to use a secure one! You may have to wait several minutes for the contents of your disk to be encrypted, depending on the size of your drive and its speed.

Don’t lose your password! If you do, you won’t be able to access any files on the encrypted drive.