I've got a 100 GB external hard drive hooked up to my 2009 Santa Fe. It takes a few minutes to start playing music (I guess the radio is reading the entire hard drive first?), but once it starts, I can quickly and easily access my entire music collection. I will caution that you have to be careful with folder and song names so that it's easy to find what you're looking for.
To play all files on the flash-drive randomly, press and hold the RDM button for 1 second. The 2011 Sonata Owner's Manual states on pg. 4-119:Could you elaborate a little on how to organize folders? I would like to be able to plug in my flash drive and play all of the music on random. Do I have to put everything in the root to do this?