Let me be the first to say Welcome To NZBDrone Project and the NZBDrone Forums
Have you tried adding it as D:\ a good way to tell if that is working is all your subfolders will start appearing (assuming you have any)
the above is for a local drive (meaning same computer NzbDrone is on) for a network drive this is a little different
if the computer that has the network drive is also a windows machine you can enter it as
\\ COMPUTER NAME \ FOLDER \
(Don’t add the spaces though i just did this as when all together it was hard to see the slashes here is how it should look really replacing the computer name with the networked pc’s share name and folder with the folders name of course)
\\COMPUTER NAME\FOLDER\
you could assuming the PC has a static IP try adding using the IP Address instead of Computer Name so
\\192.168.1.1\Folder\
Doesn’t have to have capital letters either this was just for demonstration
Update: after writing this i tested it on mine to make sure it does in fact work and yes it does but only when using computer name not IP Address so try that first
if you have any more questions or if this works for you please post here and let us know if it solves your issue so it may help others in the future i will mark as solved as well