Another thing, you know how sometimes a USB stick needs some time to un-mount? This non-sync'ed data (say, a copy of a big file) eventually has to be written to the physical disk, and through the little pipe, takes time. Find out how much of this "dirty" buffer you have yet to physically write to the device, while giving the kernel 'hints' to sync the dirty blocks quickly, with:

    while true; do sync && grep Dirty /proc/meminfo; done

Lots of gold to be mined from /proc/meminfo .. bonus points for wiring it up to gnuplot so you can get a real picture of the dynamics of the Linux kernel .. ! :)