The source box is used to select the source of the copy or verify function. In the screen shot above, the source selector has been popped up, and it shows the "Choose DDP Folder" menu item plus all of the available tape drives, in this case one actual drive and 3 tape emulators.
If a drive has already been chosen for the destination box, or is still busy performing a previous copy/verify operation, it will not be shown in this list.
Choosing the DDP Folder will put up a system file dialog that will allow selection only of valid DDP images, consisting of an enclosing folder (usually named with the DVD disc name) containing a Layer0 folder and a Layer1 folder if it is a DVD-9.
In the case of a DVD-9, it will select Layer0 in the layer popup, which you may then change to Layer1 to get the second layer.
Choosing the first (blank) entry in the popup will empty the source box.
You may drag a valid DDP folder from the desktop directly to the source box.
You may also drag a tape drive directly from the tape drive list to the source box.
Only tape drives containing a valid DDP image layer will be shown in the popup list.
Once a tape drive is selected, it will not be available as a destination.