Your external hard drive's cable should be connected firmly to both your computer and the external hard drive, and the external hard drive should be in a position where it doesn't create any tension on the cable. Click the Apple logo in the top-left corner of the screen. A drop-down menu will appear. It's in the drop-down menu. Your Mac will begin to restart. Once your Mac's screen goes black, you can start holding down the ⌥ Option key. You'll do this until the Startup Manager window opens. This window, which will appear in the middle of the screen, will have a list of your available boot locations. Click your external drive's name in the Startup Manager window. Click the upward-facing arrow below the external drive. Doing so will prompt your Mac to begin booting from the external hard drive. Once your external hard drive is selected as the boot location, your boot program or service will open; follow the on-screen prompts to open, install, or set up the program or service.  For example, if you're installing Linux, you'll be asked to select a language and country, enter your user information, and so on. If you already have an operating system installed and set up on the external hard drive, confirming your hard drive selection will open the operating system.

TL;DR in Vietnamese:
Bạn cần chắc rằng ổ cứng gắn ngoài đã kết nối với máy tính Mac. Mở trình đơn Apple. Nhấp vào Restart… trong trình đơn thả xuống. Nhấp vào Restart khi được hỏi. Chờ cho màn hình tắt. Nhấn giữ phím ⌥ Option cho đến khi cửa sổ Startup Manager mở ra. Thả phím bấm khi cửa sổ Startup Manager hiện ra. Chọn ổ cứng gắn ngoài của bạn. Xác nhận sự lựa chọn. Tiến hành theo chỉ dẫn trên màn hình.