I'm not sure how you know your drive is being read. If you're listening to sounds it makes, not all reads make sounds. An activity light either on the unit or on the screen would be more accurate.

When someone uploads from you they are reading from your hard drive unless you have a large amount of memory and small files. The application might cache the files themselves in RAM, but I don't think that's likely.

Searches on the other hand are done almost entirely in memory. Queries come in pretty fast so the speed of RAM is necessary to keep up. When starting the client up the hard drive is read to figure what is shared but once the filenames are loaded searches are done in RAM.