Here are some options for disk encryption software for encrypting USB disks:
Windows: BitLocker: Built-in disk encryption for Windows Pro that allows you to encrypt USB disks with a password.
macOS: Disk Utility: Built-in macOS software that allows you to create encrypted USB disks for Macs.
Linux: GNOME Disks: Linux software that allows you to create encrypted USB disks for Linux.
Cross-platform: VeraCrypt: Cross-platform disk encryption software. This is a good choice if you will use your USB disk in different operating systems, or if you're using a version of Windows that doesn't come with BitLocker.