The importance of backing up your EndNote library cannot be overstated. EndNote can facilitate many of your reference and citation management tasks, but this is true only if you protect your EndNote library, which is comprised of a ".enl" file and its associated "*. data" folder.
For example, let's say you create an EndNote library called "Cognitive Dissonance." Your library would consist of a file called "Cognitive Dissonance.enl" and an associated data folder called "Cognitive Dissonance.data"
Both the ".enl" file and the ".data" folder must be backed up to protect your EndNote library. The easiest and safest way to back up both the ".enl" file and its associated ".data" folder is to use the EndNote Compressed Library command to create a single compressed file. Be sure to save the compressed file (it will have a ".enlx" extension) to a flash drive, an external hard drive or even a cloud drive. Keep the saved compressed file apart from the computer on which installed EndNote.
It is possible to use Windows Explorer to make your backup copies, but you must remember to copy both the ".enl" file and the ".data" folder. However, I strongly recommend that you use the EndNote Compressed Library command to protect your EndNote library and data.
The following video uses an older version (version X7) of EndNote. However, the backup process, using the Compressed Library command, is essentially the same for the most recent version of EndNote (version X8).
EndNote: compressing and saving your library from the AUT Library, New Zealand