I have tried both ways. I found little difference in sound quality between external and internal. The internal uses a SATA connection, and I believe provides more current, but any contemporary 2.5 drive should be adequately powered by USB Bus current. IMO, the sheer practicality of an external drive more than made up for any subtle benefits from internal drive location. it's so much easier/faster to disconnect your external drive and attach it directly to your computer for DB changes, taking advantage of fast write speeds of my SSD, and USB 3.0 connections.
There is no impact on warranty with internal drive. Bryston publishes full instructions for consumer internal drive installation.
Discussion a while back on internal vs. external drives here:
http://www.audiocircle.com/index.php?topic=110296.1400