External hard drives and sound quality

I've just about filled up the internal hard drive on my Macbook with music files and am now looking at external hard drive options. Was wondering whether folks report any difference in sound quality when playing files from an external drive versus the internal?

I'm especially interested in hearing people's experiences using wireless hard drives. An Apple rep told me it would be no problem, as the hard drive wouldn't directly interface with the USB output, but I of course always like to be skeptical of anything an Apple rep says.