Although you can back up your Zotero library with Zotero Online, things can sometimes go wrong with cloud based storage. Similarly, other errors could also happen to your Zotero library stored on your computer. So, it is important to back up your Zotero library regularly. Try the following steps to save a backup library on an external device.
Copy your Zotero directory
Save your Zotero directory to an external device
You should back up your Zotero library regularly.
Find more details about Zotero directory location on the Zotero website.
If there is a problem with your Zotero library, it is possible to restore your library from an earlier back up copy. Follow these steps:
Find more restoring options on Zotero's website.