I had some similar problems, and it stemmed from rsync not starting. I’m not sure what the root cause is, but it looked like the environment variables in the startup script were not being set. I simply went into the startup script and expanded the “APP_DIR=$3” to “APP_DIR=/mnt/DroboShares/Drobo/DroboApps/rsync” (verify the explicit path on your droboshare, I’m doing this from memory…).
Once rsync would start, I was able to use the following command sucessfully:
rsync -arP /Users/bruceartman/Music/AmazonMP3/ root@droboshare:/mnt/DroboShares/Drobo/MusicLibrary/
You may need to have teh dropbear ssh package installed so that you can login, but that is easy to do as well.
Hope this helps!