Did you extract preserving permissions (typically -p option with tar)??? Your applications under /media/mmc1/debian/debian/{bin,sbin,usr/bin,usr/sbin} are probably not executable.

Debian usually puts it in bin/su, so ls -l /media/mmc1/debian/debian/bin/su and see if it has -rwxr-xr-x or similar. It may be -rwsr-xr-x if it has set uid enabled.