Time machine secondary backup - OS X keeps complaining about backup disk not available

I have a Time Capsule, which I use for regular backups of my Macs. In addition to this, I bought two external disks to use for offline backups. I have used Time Machine to do this, but it keeps complaining that

Backup Disk Not Available

when the drive is plugged out (stored off-site). I have no found a way to silence this message, but other than that, it works fine. The message keeps popping up all the time and when I dismiss it, it pops up again after a while (I guess at the next backup time).

Keeping extra backups off-site must be a common use case. Is there some setting in Time machine that I am missing? I realise that I could just manually rsync the entire disk over, but I like to avoid the manual work (I have to remember paths, exclusions and/or create scripts), and Time Machine is AFAIK essentially just a frontend to rsync anyway.

Has anyone got a neat solution to this problem? (Apart from cloud backups and proprietary third-party backup software, both which I already know of).