How to deal with an I/O bottleneck to disk?

A site I'm working with has an I/O bottleneck.

They're using a static server to deliver all of the pictures/video content/zip downloads ecetera but now that the bandwith out of that server is approaching 50Mbit/second the latency on serving small files has increased to become unacceptable.

I'm curious how other people have dealt with this situation.

Seperating into two different servers would require a significant change to the sites architecutre (because the premise is that all uploads go into one server, all subdirectorie are created in one directory, etc.) and may not really solve the problem.