I have tried several methods:
a) 250GB Western Digital MyBook USB drive via Oppo DV-970HD's USB port
b) SD card connected to Oppo DV-970HD's card reader
c) Linux laptop to 990 via USB

The most convenient (in my opinion) is the SD card via the Oppo DVD player because it requires no additional connections and I can take the music with me on my PDA. However, space is an issue, so I only keep my "A" list in my SD card and I am constantly updating it.

I am pretty hooked on Amarok for managing my mysic library and that means the laptop, so that's still my second choice. However, hooking up my external USB drive via the Oppo DVD player let's me run a slide show with images on the same drive while listening to music, so that's my preferred method when entertaining.

So, just like Demus and myself have already found out, having a DVD player with a USB port is pretty handy - not to mention cool.