Where to store your Endnote library, or not to store it
The EndNote library is a database and can thus be corrupted. Further, the EndNote Synchronization process is sensitive to the location where the library is stored.
If the storage location is a network drive or an external drive, this can pose a latency issue with saving and sending information which can disrupt the library. Even more disruptive can be ANY process that attempts to copy EndNote Library files while the EndNote library is open.
This includes any backup process that dynamically scans, and transfers data. Backup processes like Time Machine, Backblaze, Carbonite can perform this way. As does most synchronization services: iCloud, DropBox, OneDrive, SuiteSync, GoogleDrive, Box, etc.
If the EndNote library is stored in a hostile location, Sync is eventually going to collapse as the library becomes more and more damaged by conflicts during EndNote’s save process.
The solution is to save the library in a location which is not hostile (local and not synced by anything).
The above does not apply to Endnotes included sync functionality.
Where to store the library
Alfasoft cannot tell you where to store your library as the set-up of every computer varies.
All we can do is explain where you should not store your library, and why.
Basically, do not store your library in a location that is not local on your computer or is in any way synced by third parties.
The technical reason is that EndNote needs to save to several files in a specific order. If one of these files are simultaneous synced (locked), then EndNote cannot save to it and the library may be corrupted. Always make sure your library file is local to the system’s hard drive, and not on a network drive, or in an iCloud, DropBox, Onedrive-style synchronized folder.
Read a more detailed technical explanation in this article Technical reason to use EndNotes sync functionality and nothing else.
For specific Mac considerations (iCloud) see this article EndNote, iCloud, Sync and storage location