How to boot from SD/USB using internal storage for data

I found this post by kszaq to be very useful, as it combines the convenience of dual boot with speed of internal installation while retaining the safety of booting from external media.

A word of caution:
Be aware, as you would have heard before ’with power comes responsibility’ and altering the default storage location of these builds is no different.
Once you have employed any procedure that changes the default partition layout of these builds, it is entirely your own responsibilty to remember not to insert a USB / SD that also contains a STORAGE partition, as it may cause complications should two instances exist in the same setup.
Always create an external backup before any such procedure is attempted