If I run WinDirStat I can see Eraser's overwritten files. They are typically in 260MB blocks. If I delete these overwritten files will it expose the old files that Eraser had written over?
The answer is no. Even if you manage to remove the overwritten files, you will not be able to recover or access the old files. This is the way Eraser works: overwrite old files with new content to make recovery impossible.