Ian is trying to back up his iTunes folder with a flash drive, but the drive turned out to be too small. How can he tell how large his library is? Leo says to open the iTunes program and look at the bottom of the window. That will give him a general idea of how big the media folder is and how much music it contains. He can also just right click on his iTunes Media folder and select "Get Info."
It may be a better idea to get a USB external drive to back it up. He could also invest in iTunes Match, which for $25, Apple will back up all of his music in the cloud and upgrade it to AAC.
Backing up is important. Check out the DAM Book - 3-2-1 backup by Peter Krogue. The idea is to have 3 copies, on 2 different formats, with one off site. By doing that, you are guarding yourself against the worst case scenario.