So I've decide to give an usage to an old PC siting at home.
I've followed couple of how-tos on the Internet and installed motion and Dropbox into an Ubuntu installation so that I could see what's going on home while at work (from my laptop) or even when I'm away (via android, of course).
Sadly, as I came to knew 2-3 days later, the last 30GB of available memory from my laptop were gone - I found out because Dropbox warned me it could not sync files anymore due to lack of disk space...
A bit of research and I found out that the one to blame was .dropbox.cache:
So, of course, I've deleted the files inside .dropbox.cache. But it's been only 2:30h since then and, again, I've only 21,83GB left, meaning .dropbox.cache is actually eating my HD on an average speed of 3.2GB per hour.
I'm not sure if this is a Mac-specific issue, or if it might be somehow aggravated by the constant transfer of small files (pictures) generated by motion, but anyhow it looks like there are a lot of people complaining about.
So I guess I'll have to switch to a backup/remote storage alternative to this setup.