why isn't "Modified" date updated?

[dnanian]

The "Modified Date" is not part of the files in the file system, and thus isn't updated when those files are copied...and the system doesn't track changes to the file system and update the modified date. So... it's just the way the file system works, Dan.

If you want to keep track of when the copy was made, you can use an "after-copy" shell script to touch a file at the top of the drive (for example). Of course, that means the copy isn't "unchanged" from the source, but one file there won't make a difference...