Below is a screen capture of the Microsoft Windows Live Mesh, part of the free Microsfot Live software.
This is for cloud/internet synchronisation. For removeable drives, I use syncback.
Both software are not completely safe or usefriendy. I only just manage to figure out some important points:
1) Don't let it tell you which DEVICEs to sync, i.e. do not click the button indicating the remote devices. Otherwise it will create another folder in your personal folders(MyDocuments). Go to your remote device one by one, then select the folders that you want. The automatic selection may be convenient, but it will create it at a different folder, unless your PC does not have any similar folder that you want to sync, and you want to keep your documents in your personal folders only.
2). Make sure they are all online for a long time, before you touch the files. If not online, some files may not be in sync. To make sure you have the latest version on all the time, use the free SkyDrive. I got 5Gbyte. Sync to skydrive as well. It will become an intermediate device that will have the latest versions. Also a backup in case your files are missing.
3) The danger of synchronising live is that if one version is corrupted/deleted, it will be deleted in all other versions. That is why it is still advisable to have another backup that are not all synced live. Unfortunately, it will mean that it may be out of sync. It happened to me a few times during lectures.