I have had some problems trying to set Drakbackup to do differential backups. I told it do do differential backups using the user interface from PCC and when by external hard drive became rapidly full I discovered that it was doing full backups instead.
Examination of the drakbackup.conf file showed that it had enabled both differential and incremental backups. This obviously confused it and it did a full backup instead.
Asking for incremental backups gives the correct .conf file.
Asking for full backups sets the flags for differential backups!
I have tried this now on two machines with the same result so I think there is a problem with the user interface. If in doubt edit the .config file manually!
(using PClos 2009 fully updated with KDE3.5)