As I've mentioned in the past, I do use Dropbox but in a rather different manner to what I see others doing. I use a combination of Dropbox and Allways Sync.
My main Calibre libraries are stored in My Documents; I then have Allways Sync set to copy from My Documents to Dropbox; in turn Dropbox replicates to the cloud, and my Laptop gets a copy from Dropbox.
I then accept that the Laptop will in effect be a read only copy, so just copy out of Dropbox into it's own Calibre library.
Never ever do I copy from the Laptop's library back to Dropbox.
Sure, this means I can only make permanent changes to my libraries from the main PC, but it also means I can use Calibre on more than one machine.