If it’s similar to the Sansa Connect, and I remember correctly, the microSD card is only read upon power up (not waking up from sleep), unless a change has been made to its contents. The amount of time to do the refresh depends on the number of files on the card. I had a 8GB card loaded with ~256kbps MP3s, and it took several minutes to refresh.
The Fuze may be different, though.
