We know that more than one external is needed as these devices tend to stop working without warning. Just read the CNET Storage Forum and the pleas for help there.

So best would be 3 of whatever model you want then sync them when you can. Since BEST is not defined I'm going with you want 3 of the models at the next link.
Bob

http://www.amazon.com/Shadow-trade-External-Portable-Version/dp/B005KT2H0W/