Scheduled backups with external drive, not always connected

I'm trying write backups of my laptop weekly, to an external hard drive, which is not always connected. However, it doesn't seem to be working. I believe this is because the scheduler is triggering when my external HDD is not connected.

After this, it goes on believing that the last backup was "today" or whatever, when nothing has been backed up at all. Ideally I'd like failed backups like this to simply wait until they can succeed and start again.

At the very minimum, I'd like to be able to set the exact date/time of the backups so I can make sure they're at a time when the laptop is likely to be connected to the external drive (ie, schedule it for a time when it's going to be on my desk).

Can you give me any advice please?