I have the same issue. It is annoying, but I also suspect it contributes to the over all system lag that I am experiencing. In my case it is a windows share that continously "re-presents". This in combination with the problems I am having with my previous post "Media HDD" leads me to believe there are still issues to be worked out in the media detection scripts.
I also posted a question about video lag on my system. Reading the above I guess that could be origin for the lag. If only I knew what do do about it.