To create a manual back-up of your library, use Windows Explorer (or Finder for Mac) to navigate to where your EndNote library files are stored.
Select/Highlight both files - your EndNote library (.enl file), AND it’s associated Data (.Data) folder. Right-click and Copy both files, then Paste them to a different folder on your computer, and/or to a USB drive or other portable storage device.
Note: It is essential that you copy the data folder as well as the library file, otherwise the back-up of your EndNote library will not function properly.
The University Library does not recommend using EndNote libraries with cloud-based services, such as Dropbox, iCloud, Google Drive, Sharepoint, etc, due to synchronisation issues with these services that can corrupt your EndNote library if you access/update it directly from the cloud.
Thomson Reuters note on their website that EndNote was not designed to be compatible with cloud storage, and recommend:
A number of users on the Thomson Reuters forums have reported problems with corrupted files from use of active libraries with cloud-based services.
Other universities have also advised users not to save active EndNote libraries to cloud storage.