Backup plan "Currently unavalable folders" non-existent folders greyed out

Hello, I changed computers and now the drive I wish to back up has a different drive letter (D: to E:). I went into the backup plan and changed the files and folders to backup under the what to backup tab. However the files and folders from the former hard drive D: are reported as unavailable, but when I change the files to backup to the new drive letter, the files from the old drive don't appear in the list of what to backup. I have no clue how to delete them from the list, since they no longer exist. How do I make this error message go away?