How will Deja-Dup operates when backing up to an external USB drive?

I want to set up regular backups, and deja-dup seems like a nice tool. However, I want to put my backups on an extension USB drive that I have, not on a remote network location. Naturally, this drive is not always connected. If I configure deja-dup to backup to a directory on this drive (e.g. /media/extention/backup), what would happen? Will it prompt me to connect the drive when it is missing (the desired behavior), or just fail silently? Is there some way to tweak it to do so?

I can roll my own cron-based backup script that checks if this drive is mounted, but I would really prefer to use an existing, integrated tool.

Deja Dup will wait for you to connect your external drive. So, when your external USB drive is not connected, Deja Dup will show a Notification saying that "Backup will continue when <external USB drive's partition name> gets connected."

So, when you do connect your external drive, Deja Dup will start backup automatically. Otherwise, it will wait in the background till the drive is connected.

The only downside to this is it will nag you constantly until you connect the USB drive. I set this up on my daughters laptop as a manual step.

I configured dejay dup as desired (daily) but left 'automatic backups' OFF. I then made a script file to run deja-dup --backup and pointed a launcher to it. She now runs the launcher after connecting the USB drive. The first backup is long/intense but subsequent backups are short/quick.