/Volumes/HDDbackup are internal drives then your crontab should be:
00 22 * * * rsync -av /Volumes/HDD1/ /Volumes/HDDbackup
If one or both drives are external drives then you need to keep the external drives mounted when the user logs out. Keep in mind that macOS usually mounts external drives as
noowners (set to ignore permission). You should consider enabling permissions on any external drive with the proper access for that user.
Then set the preference file-
sudo defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool true
Note: I didn't test whether this preference file works in Sierra.