I have an Ubuntu 12.04 server on my old laptop. Its HDD (160 GB) is too small for the file storage I need, so I'd like to attach a couple of external HDDs to it (through USB ports). The data I'll keep there is rather valuable to me, so I'd like to organize a software RAID (RAID 1) on 2 external HDDs. The only solution for this problem I found on the net is described here (and it's said that using external drives is not recommended).
How should I do it then, and what is the difference compared to installing it on internal drives? Do I need external drives that are exactly the same (model), like for a hardware RAID? Or is it a really bad idea? If so, do I need not a RAID, but a backup solution or a NAS, attached to my server?