Why do you think installs are going to the SD card? Android 2.1 installs everything to the internal memory (applications that is). If you want to install to an SD card, you have to wait for 2.2.
Also the path to the internal 16gb memory is /SDCard
Path to your externa SD card is /SDCard/SD
So your 16GB internal is actually called SDCard
And your 8GB external is SD
I am not sure what app you are using to copy movies and music OR what OS you are using, but you can control (from that app). Also, there is a setting on the phone for pictures and movies - controlling where they are recorded.