[5.0 Beta] Dropbox, OneDrive and Backing up with SecondCopy
One of the major hassles with Endnote up through and including the current version is that *.ENL files become corrupted if you leave them in a Dropbox or OneDrive folder. Although I have a solid relationship with EndNote support going back 25 years, I've never received a solid explanation for why this is, since it isn't the case with Access database files (which also revise the hard drive with any change). Why is it that EndNote and apparently Zotero Standalone files can't subsist in a Dropbox folder? Is there no way to make this possible? If you install your data director as recommended within the Zotero folder in Program Files, you can't use Windows restore to go back to a clean windows copy or earlier windows/programs copy, without going back to an earlier Zotero database or assuming that you can sync with the version on the Zotero service accessible via Firefox Connector. There is probably a good reason for this, but I've never heard it. I do recommend a solid workaround however: the awarding winning shareware program Second Copy (www.secondcopy.com), still supported by the same file folks. This program saved my dissertation more than once from crashes, theft, etc. With this you can do this: its manual cloning to one-way clone your data director to a mirror file in Dropbox or OneDrive which is never used for actual Zotero operations, and do the same with an automatic profile that you run only when you shut down and start-up. If Zotero isn't loaded at start-up and is closed before shut-down, theoretically your Zotero database would never be corrupted and would be backed up, protecting against crashes, theft, etc.. Perhaps I'm wrong, I'm a Zotero neophyte.
That page has all of the details for what to back up.
Regarding Windows Restore, you can set your data directory to be located in, for example, your User Folder (where Documents, Pictures, Desktop, etc. also are) in Zotero's Preferences (the Advanced pane). That folder won't be touched by Windows Restore. When Zotero 5.0 is released in the near future, the default location for the Zotero profile will change to the User folder, as well.
Any backup solution that copies Zotero's data directory will work. I use Time Machine on a Mac. LiborA's solution also works, as would any other backup solution that periodically copies the contents of specified folders to an external device.
You really, really don't want to touch your Zotero database with Dropbox, OneDrive, or other cloud sync services. It takes a lot of diligence to be sure that no Dropbox sync is ever running when the database is open in Zotero, which will cause corruption. Also, cloud servers shouldn't be regarded as a backup solution--they are for storage, but you shouldn't rely on them as your only backup. You can google around for various horror stories of people's photos, work, etc. being destroyed by a malfunction or misunderstanding of a cloud sync service.
There are various ways of making that happen safely with Zotero; the key part is that you don't place the active Zotero database into a folder that syncs between devices (i.e. the Dropbox, OneDrive, Google Drive folder). You can set up a one way sync to an oline back-up of sorts (many services offer that), you can set up a script that copies to zotero database into Dropbox et al. in regular intervals -- there are probably more options.
I kind-of follow the two locations and 3 copies model adamsmith mentioned, except my two locations are home and office (and not two separate cities, which is better). With Syncback, I create a profile that mirrors the backup copy to what is on my computer. Running the profile is a one-click task. How frequently I run the update profile depends on the intensity of the work/updates I am doing (could be once every 3 hours or once a week). This might not be the most robust or the most frequently updated model, but has worked flawlessly for me, and has been proven over multiple hard drive/computer failures which are inevitable. I could setup Synback to work in the background and continuously mirror but I never liked that model.