Head to the directory where the bank file is stored
Spoiler : Path :"C://Users/YOUR_USERNAME/Documents/StarCraftII/accounts/SOME_NUMBERS/#-S2-#-#######/Banks/1-S2-1-689689"
Spoiler : Path :"/Users/YOUR_USERNAME/Library/application support/blizzard/Starcraft II/accounts/SOME_NUMBERS/#-S2-#-#######/Banks/1-S2-1-689689"
In macOS, the Library folder is hidden by default. To open it, open finder and click the "Go" menu on the status bar at the top of the screen. Now hold down the option-alt keyboard key and a library option will appear in the "Go" menu. Then the next folder you go into is "Application Support", and so on; see the mac location above.
Copy the "Mbank13.SC2BANK" file into wherever you think would be safe or convenient for you, such as;
- Cloud Storage services (Dropbox, Google Drive, Microsoft OneDrive, Apple iCloud, etc)
- A flash drive/External HDD
- Another computer/laptop
You can use software such as "Everyday Auto Backup"(suggested by Sureiya) to automatically copy the banks file/folder at set intervals of time to another location.
To Restore your Bank
Put the "Mbank13.SC2BANK" data back into your StarCraft II installation, go to the same directory from step 1 and replace the "Mbank13.SC2BANK" there with your backup copy.
This guide was written by @Mugy I just gave it its own thread.