I want to use a different memory card for some data which is in a sub-folder designated as a source folder in a backup task on my computer. (I detach the memory card from time-to-time and put it into my EeePC when I am travelling.) I have copied the data across from the old card to the new card. I then went to edit the backup task so that it uses the new data.  I cannot change the top level folder which corresponds to the drive letter assigned by my PC to that memory card.  I want to change from G: to I: but it will not let me.  It looks like I have to delete the backup task and create a new one.  Is that right?

Yes, the current version doesn't allow changing the source folder, though you can add more source folders after it has been created. You can simply delete the backup task without deleting the backup sets on server; then recreate the backup task.