I used Windows Media Player and it worked great.  You can use .wma or .mp3 formats.  You CANNOT use a 6GB card.  I know you can use up to 2GB, but I did not try it beyond that.  Anything over 4GB is definitely out of the question.  Different technology used over 4GB.  I used mainly 1GB cards and get over 200 songs on a card recording at the best quality.  I found a small, hard case that holds 8 SD cards that I carry with me, available at Best Buy.  I can't remember how much it was, but definitely under $10.  If you need to have quick access to more than 1600+ songs with you at all times, you are out of my league.  I know I've ridden over 450 miles in a day and wasn't finished listening to a 1GB card.  Get the 2 GB cards are you have over 3200 songs in the player or in your bags and probably a 1000 mile trip without changing the card.  If you need help using Windows Media Player to record onto the cards, I can probably walk you through it.  I just traded my bike with the MP3 player last week, so now I'm wondering if there is a way to record a lot of mp3's onto a CD, or do I need to get the HA-90 again?  It's a great unit, but I think everyone that gets one has lots of questions initially that the manual does not address.  When I got mine, I contacted the company that makes it for Harley and let me tell you, they really didn't want to give me answers to anything.