I have an external USB HDD and I like to keep it synchronised with my desktop for backup reasons. I find the easiest way to do this, is to use rsync:

> sudo apt-get install rsync
> rsync -avu /source/ /dest/

For example:

> rysnc -avu /media/externaldisk/ /home/chris/diskbackup/

Will increment copy everything (ignoring older files) from /media/externaldisk to my /home/chris/diskbackup folder.

The options are:

• a – Archive mode – Will retain the folder/file properties, such as owner, permissions, etc.
• v – Verbose – Output the progress to the screen (so I know its still working)
• u – Update – Only update files that have a more recent file modified date.