I have a 500gb WD Passport that I use for backing up my computers, and also for use with my Wii.
I can easily stick a few songs on it and give it a try with the RNS if you want?
I have read about HD's not remembering where the song left off - which would annoy the hell out of me.
My RNS HD has my regular songs on it - and I have an old 32gb iPod that I keep for using with the car, and a 16gb memory stick that I chop and change songs on.