I still remember the day when my ipod broke down. Before jumping to the apple store I asked myself if I can't use my old Android phone instead (that I took over from my girlfriend when she got a new shiny iphone).
I plugged it in. It became immediately available as an external drive. I copied my mp3s over. I was done.
Please don't take this away from me...
As for the mp3 scenario: for a 1Gb mp3 player its certainly doable to just scan the contents for the files/metadata once in order to show track titles etc. The problem is when the collections of files grow by several orders of magnitude and you need instant lookup of metadata (e.g. a 1Tb photo library, which isn't even especially large these days, my camera takes 25mb photos with hundreds of pieces of metadata that can't be encoded in the filename unlike an mp3 track).