Hello Jon!
Grasping at straws, any chance you also left the original files in the directory "lists" after copying them to the new partition. After mounting the new partition on /storage/lists they would be "masked" and would not be accessible using file system semantics. But dump-like programs could still see, and back-up, the masked files. Tar should be using file system semantics, but like I said, "grasping".
I just double-checked and I'm pretty sure I did delete the original files from the directory. When I umount /storage/lists, the directory appears empty.
-- Toomas Aas
