To make a back up library, open up your EndNote Library and then click on File > Compressed Library. You will then get a dialog box where you can select what your compressed library will include. We advise that you select all references with file attachments, as this will ensure all your work is saved. Click on next and name your compressed library appropriately (you may want to call it 'My EndNote Library compressed' or 'My EndNote Library back up'). Choose an appropriate place to save the compressed library (we advise that you save it on your H drive, alongside your other libraries) and then click on 'save'.
We advise that you back your library up regularly in this way, especially if you have made a lot of changes. Back up copies are not saved automatically.