Collection disappeared
                    I've used Zotero for a year.  I moved to Zotero because I had major issues with Endnote and Mendeley, particularly with stuff disappearing.  I just recently had same issue with Zotero.  I have several layers/levels of collections in one library.  I recently noticed where one second-level collection that had probably two levels within it went blank.  The collection folder is still there, just none of the sub-collection or references.  Luckily I had a fairly recent backup in another library (I learned my lesson from past) I could use to rebuild it.  But, is there any way of figuring out what happened?  I am pretty sure it wasn't just user error (accident), since about a half dozen sub-collections disappeared, rather than just one.                  
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 Upgrade Storage
 Upgrade Storage
Library
Collection
Subcollection Level 1
Subcollection Level 2
Subcollection Level 2
Subcollection Level 2
In this library we have 8 Collections, and each has three Subcollections Level 1, and each of those has numerous Subcollections Level 2. In this case, for this Subcollection Level 1 of interest, all Subcollections Level 2 (I think about six of them) disappeared. The other Subcollections Level 1 under that Collection remained. The references were still in Collection, but no longer in that Subcollection Level 1.