Clarivate (EndNote) does not recommend using EndNote Desktop libraries with cloud based services, such as OneDrive, Dropbox, iCloud, Google Drive, Sharkpoint, etc. due to synchronization issues with these services that can corrupt your EndNote library if you access/update it directly from the cloud.
Back Up, Back Up, Back Up
Your EndNote library is saved when you create it.
This is made up of 2 files. One is the library name.enl file, the other is the library name.data folder. You will open and work with the .enl file but the .data folder will contain any PDF stored in your library, the trash file, groups, term lists, etc. It is very important and must not be removed or renamed unless you are deleting your library.
Save as a compressed file:
Select File > Compress Library (.enlx). This will compress/zip up both files into a single file (.enlx). This also gives you the option of saving or emailing it; with file attachments or without; the whole EndNote library or just the references in a group or selected references. This is the preferred option if saving to an external drive such as a USB. To unzip/decompress this file - simply click on it to open it.