Hi menzieto and welcome to cdfreaks.
Yup, agree with signals and Metallica Boy...usually a little extra patience (my impatience is often what causes my problems... :doh: ). First load the blank media disc and wait a few seconds before starting the burn. Ideally, if using 2 optical drives, load both discs...wait a few seconds prior to even opening the app.
Re: an .ISO rip
Open Fab, note at the top, the 2 drop down menus: "Source" and "Target"
Note the icons immediately to the right of the drop downs.
Near right icon = folder icon and represents file mode.
Far right icon = .ISO icon (image of disc with page overlay).
To rip to .ISO on your hdd, simply choose the .ISO icon on the target drop down...browse to desired location on your hdd and you should be all set.
(If using 2 drives and immediatley burning to a disc, just click the .ISO icon and leave your burner as the target).
To burn from a previous rip on your hdd, open the .ISO icon on the "source" drop down and navigate to desired .ISO file.
Hope this helps.