Best External Harddrive FCP and iMac

Hi:

I have an iMac from 2011 and am using FCP X to edit. I've been using G-Tech G-Drives with firewire. Can I use USB 3.0-Thunderbolt drives with this system?

Also, been putting off upgrading to El Capitan. Nervous about bugs...esp. in the middle of editing a project. Any thoughts?

Thanks!

Nicolle

Mid 2011 27' iMac has two Thunderbolt ports. No USB3 ports, only USB2.
2011 21.5" iMac only has Firewire and USB2.

Don't upgrade OS in the middle of a project.