Backup / mirroring

Following a suggestion on this forum I'm now a happy user of Genie Timeline. It's doing all I want of a backup product.

Well nearly

It quietly backs everything to the selected drive (an external USB drive in my case) with negligible impact on the running of the PC. But I'm wondering if there's a simple way of mirroring (right term?) the backup drive so I'm not dependant on that one drive. Clearly I could go Raid but that appears to waste my investment in external drives. Any simple way of achieving this? By software perhaps?

I actually do use a mirrored setup for my Genie Timeline backups. (Picked up a Linsys NAS going real cheap in Currys and stuck a couple of 2TB drives in it.) I'm not sure that there's any easy way of mirroring separate USB disks. The only way I can think of would be to use "robocopy" to keep the disks synchronized daily. How long that would take depends upon how much data gets backed up each day.