How to Install any old/new MacOS into USB Drive or internal hard disk of MacBook

- Please watch the upper ☝︎ video carefully and follow the steps. (in case the video doesn't work, press on this link) 
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keeping a macOS system installed on a USB flash driver is critical and can be invaluable if your system suddenly fails to boot. 
This may give you a troubleshooting options which permits you to recover your data or keep working on the identical copies of your data in your main hard disk. 

The above video tutorial describes: - How to Install any old/new MacOS into USB Drive or internal hard disk of MacBook.

All of theses steps have been discussed in this tutorial: -
- Where to download all official versions (old & new) of macOS?
- How to create a bootable USB Drive using these macOS.
- How to format a USB Drive to be suitable for bootable macOS
- How to format a USB Drive to be suitable as an external Hard drive
- How to install MacOS on internal hard drive
- How to install MacOS on external USB Drive
- How to solve "This copy of the macOS installer application is too old to be opened on this version of macOS."
- How to downgrade your MacBook to any version of macOS.

Briefly: - 
       It is favorable to install macOS on internal hard drives but you can easily optimize an external flash drive to be suitable for installation of macOS: -
1) first, you can download the wanted macOS system form apple website :- [N.B. you have to download the Combo version of the macOS system].

2) you must have two USB flash drives:-
     a) 16 GB (that will be used as bootable).
     b) 32 GB (that will be used as external hard drive where we will install the macOS system)

3) Format the 16 GB flash drive as "exfat" using "Disk Utility". then open the downloaded macOS system and navigate to. find a folder called "create install media" and create a bootable USB Drive as described in the above video.

4) Format the 32 GB flash drive as "APFS" using "Disk Utility" to be used for macOS installation.

5) Now, boot from the 16 GB flash drive by pressing "options key" while restarting the MacBook, the select the bootable USB.

6) your USB Drive will begin to boot.

7) complete as described in the video.....