Macossierra10126frenchiso -

If you own a valid license for macOS (which is technically free since OS X Mavericks) AND you are installing it on Apple hardware or a virtual machine on Apple hardware, Apple’s EULA generally permits this. However, downloading pre-made ISO files from third-party file-hosting sites (Rapidgator, Mega, Torrents) is legally risky. Apple does not distribute official ISOs.

: Insert the USB, restart your Mac, and hold the Option key. macossierra10126frenchiso