Just to close the thread on this one since, honestly, those other two posts left me thinking that formatting was going to be my only option and I wasn't happy with it. based on past experience, even though formatting is not supposed to mess with my apps, I could tell it was likely something might go kablooey. For example, Listen stores downloaded podcasts on the SD card and a few other apps have things on there too. In short, I just wanted to avoid a format if I could....
I used this trick: Show all files in the Finder | Software | Mac OS X Hints | Macworld
to display all the hidden files on my mac (on a mac, anything with a dot (.) in front of the file name is hidden by default.) and saw a folder called .Trashes. I opened it and the enclosing folder and found.... almost 8 GB of files, that I had deleted today! Voila.
I tried to delete them all but the computer just hung so I did a search here and found a thread which explained the problem and the solution without having to format.
Essentially, I had connected my Eris to my mac, deleted everything and, as macs do, it had sent those files to the trash. For some reason, that means that it hides all the files on my Eris' SD Card in .Trashes. To actually get rid of them, connect your phone to your mac, mount it as a drive, then empty the trash on your Mac. And there it all goes!
Of course, remember that once you empty your trash, those files are gone forever, so be sure you mean it first.
Phew. Problem solved.