Can you tell me how to do this? I used to use Mendeley before it was bought by Elsevier, and it had a useful function of exporting your entire library to a single BibTeX file automatically each time you added a new reference. I've not found a way of replicating this behaviour in Zotero (I can manually export to BibTeX, which is great, but I'm lazy and if the program can do it automatically, I'm all for it!).