Raid 1 with an external drive?

Can I use a UBS 2.0 external har drive as a raid drive?
Should I?
I want to because I have 2 external drives and want to raid 1 my 500 GB data drive.

If it is a bad plan I will pony up the $140 +/- and get a third drive.

I once tried it with one f the 80gb internal drives and an 80gb external USB. It was very unreliable, the external was hardly ever synched, and used to have to be rebuilt.

That's not to say it can't be done, but I found it wasn't worth the trouble - I'd certainly have no confidence in it.