RAID 1 on 2 external HDDs

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?

One thing that is often overlooked on a home setup is that you usually would be better off without raid / mirroring. Unless you have a need to protect data that is less than 24 hours old, you would usually be better off using the second drive as storage for your backups of the primary drive. That way, assuming data doesn't change much (where you could get use out of incrementals), you could be protected against both physical failure and filesystem or other soft failures.

If you just want to use a bunch of disks with some form of redundancy as a single volume, you could use greyhole - which seems perfect for your useage case - it supports redundancy and lets you add and remove volumes at will.

ZFS or btrfs might also be options with raid style volume management options but neither is as simple as greyhole