Final Cut Express

I have a dual-drive mirrored Western Digital WDH2Q40000N My Book Studio Edition 2 4TD external hard drive. I have been using a Western Digital external hard drive as my scratch disk for FCE. But the question is whether the above-mentioned dual-drive mirrored My Book operates well as a scratch disk in video editing due to the fact that it is continually "mirroring" the material. And that is what I hope for so that my hard work can be backed up instantly. But is it effective in video editing? Have you heard one way or another on this? It would be great if both the backup was constant and the editing was efficient at the same time.