I’m an avid user of Dropbox, and also of the fantastic layered filesystem UnionFS-FUSE. However, I found that they don’t work well together. Whenever I tried to mount a unionfs folder somewhere where Dropbox could see it, Dropbox would seemingly randomly omit files from upload. Scary! This is supposed to be my backup!
It took me a long time of trial and error, but fortunately I was able to find the magic flags to make it work. If you mount your directory with these options:
unionfs-fuse myfolder=RW ~/Dropbox/union-folder -o big_writes,large_read,readdir_ino
…then Dropbox and UnionFS-FUSE should work together happily once again!