Sitting here at the library thinking about writing (but somewhere else that is open later), I realized I hadn't tried reinstalling Dropbox since upgrading the OS. It wasn't working before. Checking, I found it's because Dropbox only works with Mac OS X 10.4 or higher, which I now have. So I reinstalled it, and it's working now!
Now I'll need to go through the Dropbox on my PC to move out things that will just take up space on my laptop, and sort through my laptop files to see what files are being doubled now.
But yay! Need for portable drives on this laptop is now reduced to "only really big files"!!!