If the external hdd doesn't have its own power supply, and is relying on the pi's usb for power, corruption of data is very likely.

I ran my pi using a Samsung 160GB usb drive for a whole day with no problems, after setting it all up. I know this is a low power drive. The next day, identical setup and there were all sorts of drive problems (stuttering audio, songs not being found in the database etc). I plugged in a usb powered hub, but the drive wouldn't work through it. My usb hub is a Ritmo cheapie, only meant for flash memory sticks, keyboard and mice, I think. Not HDD's. If you use HDD's that are in their own box with their own power supplies, you should be right.

Some internet stuff I read says that the whole pi setup needs an amp or so, and the pi requires about half of that. The usb sockets are only rated for 100mA, even though you can get more out of them. Sorry, I read all this on a page I can't find right now. But if you google "Raspberry pi power" you'll get lots of information.

ps just tried running the raspyfi from the usb slot on the home theatre setup. Powered up OK, but made a lot of low level rumbling sound come out of the subwoofer when the SD card drive was being accessed. So back to its own power supply, which I was hoping to remove.