I take a bit of issue with this; I would say that my area of expertise is with networking protocols, specifically HTTP and derivatives, rather than filesystems. Also, the 'expert' part is a bit uncomfortable.
Back in 1987, as an unemployed freakbag, I decided I wanted to be able to code in 'C'. Two years later, I took a summer course at U of D to get my feet wet. I really learned to code 'C' on-the-job at Burlington Coat and Timex and on a few consulting projects. I'd still never consider myself expert; 'C' is one of those things that, as they say, takes a minute to learn but a lifetime to master.
Thanks for the props, boss. I guess I can check 'C' off the list now.