You are doing the "umount" part correctly. The problem is (and this is indicated by the error message) that you can't umount a filesystem (on every UNIX, this is nothing special to AIX) as long as there is a process using a file in it or there is a user session with the PWD somewhere in the filesystem.

Therefore a "mount /myfs" followed by a "umount /myfs" will work, but if you do a "cd /myfs" in between it won't. Change your working directory to some other place and it will work again.

With "fuser" you can (this is why andryk gave you the hint) find out processes which use files in the FS.

As this is a beginners question you should ask it there and ask a moderator to move this thread there.

bakunin