I wanted an MP3 player with scads of room, and not a 5 or 11 ounce one, and decided that having an SD card slot in a flash MP3 Player would be my solution! Unlimited storage! I found two inexpensive MP3 players on ebay - the V@MP VP-425, 256mb and an SD slot, and another, different brand, the Astone Xinc Ultra, and my model has 2gb onboard and an SD slot! I bought them both, and went searching for some SD cards. Found a great deal on 4gb cards. Bought some.
Now both of these players can accept cards where the music on them was simply pasted in, so to speak, either while the SD card is in a reader on your computer, or where the card is in the player, and the player is attached to the computer by USB. Neither needs (or possibly even works with) music storefront software.
So I filled up my 256mb, and I filled up my 2gb, well, not really, I only had about 800 mb of my personal favorite music to put on there. But I have 21,000 country western music MP3 song files (yeah, really) on an HP 160gb HD1600S USB drive, and put a bunch on my 4gb card. Strangely, the Astone saw all of the directory names, but saw the ones toward the 'end' as empty. They were certainly not really empty. The V@MP saw more than the Astone. I made sure I wasn't confusing songs onboard with songs on the card.
Then I started writing emails to Astone. (V@MP is no longer in business).
It turns out that MP3 players have limitations as to what they'll see on an SD card. First, it's good to know that SD and the newer SDHC are not the same, and a 4gb card might have been either. (The HC is for "High Capacity" and has a different file structure.) Second, a player just will not see an entire card, necessarily. Nor will it see an infinite number of files! The V@MP seems to see a 1gb card in full. The Astone only sees 512mb. This is not a firmware upgrade, although increasing the maximum number of songs seen on the card IS a firmware upgrade, and they sent me one, and frankly, I'm scared to try it.
SO, be careful you know what you are really getting... and that it is what you need. Me? I'm sticking with playing my HP Personal Media Drive files on my computer, and loading to a bunch of 512mb cards as the spirit moves me... :)