Read the manual part which explains the times where -u works, and don't.
It has to do with several factors.. one is time-stamps.. if the files on your source disk are newer than the destination.. then they will keep copying them over and over.
Also if you are using two different file systems.. I *think* this can be a factor, because rsync will consider this a change, even tho time-stamps are the same, or older on the source.
If you have file systems that have switches which shut off the updating of time stamps to the file system (ie. noatime in linux).. then this might be an issue tooo.
Changes of sizes (which can show up over different file systems I believe) will also FORCE a copy being made.
Again, this is spelled out in the rsync manual.. worth a good read if you want to reallly utilize this awesome program