I noticed one quirk that might affect how it looks at the SD cards. Mind you this was a 2GB card, but it might affect the larger ones as well.
I found that it would only search one level of directories for music. If it found no music in the first level of subdirectories, it gave up.
So this works:
SD Card
-->Album1 (folder)
---->Song1.mp3
---->Song2.mp3
---->Song3.mp3
-->Album2 (folder)
---->Song1.mp3
---->Song2.mp3
---->Song3.mp3
But this does not work:
SD Card
-->Music (folder)
---->Album1 (folder)
------>Song1.mp3
------>Song2.mp3
------>Song3.mp3
---->Album2 (folder)
------>Song1.mp3
------>Song2.mp3
------>Song3.mp3
In other words, if the 'top' directory of the SD Card contains music, or contains a bunch of folders which themselves contain music, then it would be fine. But if the 'top' directory had no music, and the next level of directories also had no music, then it would simply give up and stop searching -- even if there was a hundred song files in a 3rd level directory.