This is probably a rare issue, but sometimes when I’ve copied a set of wallpapers to the SD card on my HTC Desire, the Gallery app simply refuses to update itself and reflect the changes. It can be mildly frustrating, and even a reboot won’t help or clearing the cache of the Gallery. You can erase the data of Media Storage to force a refresh, but that’s an inconvenient solution since all the media libraries then have to be rebuilt from scratch.
Copying songs to the memory card and then directly trying to play them in the Music app can also result in the new tracks being inaccessible. Since Android doesn’t provide a way to manually refresh the media libraries, you just have to wait until the system decides it’s time to update them. It usually works just fine, but I’ve been frustrated enough times to figure out a way to solve it.
SDrescan is a tiny app by a developer called Benjamin Rosseaux, and it simply invokes Android’s media scanner in order to add new files on the SD card to the various libraries. A quick and simple solution whenever Android falls short.