I'm using rockbox on all of my players, and i use three different microcd cards. Sometimes they either a) freeze when being ejected from the computer or b) freeze the player at some point during use, boot, etc.
I was thinking it might be the files my mac (osx fully up to date ML) generates on the cards. The hidden system files, such as .trash, etc. etc. Should those post any problem to the players when they are doing anything? Perhaps cause them to get stuck scanning unknown file types or something? I've found no easy way to eliminate these files permanently...