Mac Sierra, High Sierra, and Mojave Users
MacOS versions 10.12 (Sierra), 10.13 (High Sierra), and 10.14 (Mojave) contain features to save the contents of the Documents folder in iCloud and to "optimize" storage by storing less-used files only in iCloud online. EndNote libraries stored in iCloud, even when stored on both the hard drive and online, are subject to corruption over time. If you choose to store your documents in iCloud, we suggest creating a separate folder in your home folder and saving your EndNote libraries there. Do not store EndNote libraries in iCloud or any other cloud-syncing folder, and do not store EndNote libraries in the Documents folder if you have selected the option to store documents in iCloud.
You can find more information on these features on the Apple support site at these locations.
General Safety Notes
EndNote libraries are made up of two parts: the .enl file and the .Data folder. These two parts must be kept together, both are needed for the library to work correctly.
EndNote libraries should be kept on your computer's hard drive. Storing and editing libraries on a network drive can lead to corruption and performance issues.
EndNote libraries should never be stored in cloud-syncing folders such as Dropbox, OneDrive, Box, SugarSync, etc. Syncing folders corrupt EndNote libraries over time.
Uninstall your previous version of EndNote before installing a new version.
References in EndNote libraries can be synchronized between desktop and laptop computers, the online library, and an iOS device using EndNote Sync.
Complete EndNote libraries can be shared with other EndNote X7, X8, or X9 users through EndNote Sync and Library Sharing.