How to sync files between the computer and an external hard drive?

There are thousands of photos on the computer.  Now I have copied them on to an external drive.  I suppose I can use the Sync Centre to sync them.

I can't see the external drives in "Set up new sync partnerships".  I suppose the external drives are not networked drives.  That is why I can't see them.  Am I right?

I read up http://windows.microsoft.com/en-au/windows-vista/sync-center-frequently-asked-questions and http://windows.microsoft.com/en-au/windows/working-with-network-files-offline#1TC=windows-7.  They don't help me set up the sync partnerships.

I intend to have the sync happen manually because the external drives are not always connected.