Really. That's quite odd, I've run that on a Pi B before. I'm running my torrent server right now on an Odroid C2, and it's all USB. The Odrod also runs my Squeezebox server.
The only thing I could suggest is that you may want to use a native format like EXT3/4 for your drives. Sometimes, non-native formats can be a little slower. Or, if you were using drives powered by the Pi, get a powered hub or externals that have their own power supply. I know the Pi says it can power stuff like that, but that's up for debate at times.
I see you did say power issues, so yeah. Get a powered hub, that probably will solve your problems.
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