The volume e.g. /dev/sda8 is being used by processes and processes owned by users. So to find the list of users who are accessing the volume, first you need to find out the list of processes which are accessing the volume, then you can check the ownership of the processes. This command may help you to achieve the same:-
# lsof /dev/sda8|awk '{ print $3 }'|uniq
I hope this is what you was looking for.
P.S.:- Please make sure, if you are asking a new question in the forum, please create a new thread instead of hijacking the existing thread. It helps avoid the confusion and better visibility of the discussions.
Thanks
Rajveer Singh