Any reason you want to use 'dd'? Trisquel has an excellent backup utility (in
"System settings"):
incremental backup (only backup what has not been backup-up yet)
compressed backup (faster and save space)
integrated with the file manager (you revert to any previous version of the
file/folder in about three clicks)
periodic backup (or a notification if the external drive is not plugged)
very easy to use (choose where to backup and then use the graphical utility
with two buttons: "backup" and "restore"
possible encryption
...
