I never did get around to trying the batch files yet, but I did get some success with using the Removable Folders option. After much mucking about and re-setting the drives up one by one, it's pretty much successful.
There's only one thing now that eludes me. It looks like the backup jobs are broken down into smaller files, and they're not uniform size (ie. 3GB, 9GB, 15GB, etc.). So, say several 'media' pieces are created on a drive for a single backup. It started that one HD in particular (I think) would run out of space. So I'd delete the files from it, 'scratch' the media in BE, and re-run the job. This went away, but now it comes up with a database corruption error every so often. Again, I just delete the older files on the HD, 'scratch' the media in BE, and it works again for a few weeks.
But why is it slicing the backup jobs into smaller bits? I haven't changed any settings for the folder to be different than they were before, and have specified 100GB as the max single backup size.