Hi Christine,
That’s awesome, and so quick! I’ve installed 1.6.21.0 and tested what happens when I disconnectand reconnect my NAS.
When my Dropbox is up-to-date, and I disconnect the NAS, what happens is precisely nothing at all. Dropbox remains updated, I don’t even see any work being done by either Dropbox or Boxifier.
When I then reconnect the NAS, the same nothing happens 
All of this is perfect, of course, exactly the behavior I want as a user: if the external folders are unavailabe, don’t bother attempting sync.
When the NAS is disconnected, and I change something in one of the Boxified folders in Dropbox Web, Dropbox does attempt to sync. I don’t know if that is by design, nor if it can be fixed/avoided. In any case, I can easily live with that. A handful of actual Dropbox-sided changes take less effort for Dropbox to sync, I assume, than repeated attempts to sync all content in the external folders.
So, my conclusion so far:
Unnecessary sync attempts of disconnected Boxified folders: fixed!
Doomed-to-fail sync attempts of server-sided changes to disconnected Boxified folders: it would be great if those could be eliminated as well, but the firstf fix is a major improvement already. And I imagine Dropbox initiates those without Boxifier’s intervention, so I can understand it if that’s not someting Boxifier can fix.
Thanks again!
Floris
PS: What’s the status of 1.6.21? Can I keep using it? Beta, pre-beta, release candidate?