Yes, @Wim_Jansen is correct - now you write the script you want to run to do the “is the USB drive connected?” check and have that script spit out the correct return code to tell Duplicati whether to continue with the backup, abort with notification, or abort silently.
I know - it’s not as simple as we’d like, we just haven’t gotten there yet. 
If I get some time I’ll try to post a sample windows batch file Until then, if you really feel like reading more on this, here’s an older topic that I think covered the subject fairly well.