Load sharing mirrors are read only unless you are accessing it via the admin share.
Check out page 57 (7.2 Accessing Load-Sharing Mirror Volumes) of the following TR:
SnapMirror Configuration and Best Practices Guide for Clustered Data ONTAP - http://www.netapp.com/us/media/tr-4015.pdf
"By default, all client requests for access to a volume in an LS mirror set are granted read-only access. Read-write access is granted by accessing a special administrative mount point, which is the path that servers requiring read-write access into the LS mirror set must mount. All other clients will have read-only access."
When you are accessing the admin share for write access, you are accessing the source volume.
"After changes are made to the source volume, the changes must be replicated to the rest of the
volumes in the LS mirror set using the snapmirror update-ls-set command, or with a scheduled
update."