/data/ISO -> /data/IDE/ISO
So /data/IDE/ISO exists and you want to make a shortcut /data/ISO?
from man ln:
ln - make links between files
ln [OPTION]... TARGET [LINK_NAME]
ln [OPTION]... TARGET... DIRECTORY
ln [OPTION]... --target-directory=DIRECTORY TARGET...
Create a link to the specified TARGET with optional LINK_NAME. If
LINK_NAME is omitted, a link with the same basename as the TARGET is
created in the current directory. When using the second form with more
than one TARGET, the last argument must be a directory; create links
in DIRECTORY to each TARGET. Create hard links by default, symbolic
links with --symbolic. When creating hard links, each TARGET must
ln -s /data/IDE/ISO /data/ISO
what happens, why cannot you link them?
You may also consider mount --bind where you mount one directory to another place. I don't know what are the advantages of the second approach.