When the autoplay comes up there should be a checkbox for "always choose this option when I plug this device in"
If you choose "do nothing" and check the "always" box that should fix the problem, I think. Unless it's seeing it as a different device every time.
Windows 7 doesn't seem to have that option like Windows XP does. I just poked around for a few minutes before going to work, but it seems that Windows 7 only has an overall ON/OFF switch for AutoRun/AutoPlay, or On/Off for different file types, but I couldn't find anywhere that one could adjust AutoRun/AutoPlay settings for individual USB devices.... Odd, that.
Loveshack, you could try going to Control Panel/Hardware and Sound/AutoPlay and changing settings (like setting Mixed Content = Do Nothing) there, but that probably will also affect any other USB device you happen to plug in.
I wouldn't think plugging them into different ports would matter, but if you had any software installed with the devices, maybe reinstall the software might help..?
Oh, there is a listing for Devices (toward the bottom of the page) under Control Panel/Hardware and Sound/AutoPlay, but nothing shows up in that list for me, even when I have my little 2GB Kingston or my external HD plugged in; you might try looking in that list and seeing if your external drives show up there, you may be able to change individual device settings if they do.