To back up your Zotero library, you will need to locate the Zotero Data Directory, where your Zotero library is stored on your computer.
To do this, open Preferences via the Gear menu.
Next, select the Advanced tab and then the Files and Folders sub-tab.
Now, select the Show data directory button.
Your data directory will now be open.
Reduce, BUT DO NOT CLOSE, the data directory window.
Next, close Zotero and Firefox [It is not enough to just close the Firefox window, you must exit Firefox via the top menu.]
Now, go back to the data directory window. You may need to move to the next higher level in the file directory in order to find the Zotero folder.
Once you find the Zotero folder, copy it.
Finally, paste the Zotero folder to another location, such as a flash drive.
You have successfully backed up your Zotero Library.
**Warning: Exporting and importing your library (for instance via Zotero RDF) is not a recommended option. None of the available export formats allow for a complete transfer of your library data.