Time machine on a USB share?

Hello everybody!
I bought a few days ago the PR2100 NAS, and I’m very pleased with it so far. This one is my first NAS, and I need some insight from more experienced people.
I was wondering if its possible to attach through USB a Mybook duo to the NAS, and use that unit for Time machine backups and making use of the Duo’s Raid1. I read somewhere that you can have problems because the NAS is trying to index.
Does anyone have experience on a setup like this? Or it is better to acquire a cheaper NAS, like a Mycloud Home for his purpose?

My Cloud doesn’t allow to make Time Machine share on external USB connected share. You can copy backup data in USB share but not create Time machine backup share in a another USB share.