I just installed stretch to a fresh 2T HD. letting it autopartition and
format for separate /, swap, /var and /home partitions. But I didn't let
it overwrite the grub on the 1st drive it was/is booting wheezy from.
I figured I'd mount it to wheezy and copy over my personal stuff, like an
email corpus well over 15GB reaching back to 2002.
But I can't mount much of the drive, / is all that will actually mount,
because the 2 versions of ext4 are incompatible, nearly all the mount
and e2tools can't touch the installers ext4 file systems.