Can't upload ISO to shared storage

I've got an OmniOS Napp-IT box connected via NFS to a 3-node Proxmox cluster. Everything works great, until I try to upload an ISO file to the system via the web interface, where I get a permissions error.

starting file import from: /var/tmp/pveupload-651fa0b939061fb69fd14d1578e7006e
target node: proxmox1
target file: /mnt/pve/SanBox/template/iso/ubuntu-14.04-server-amd64.iso
file size is: 591396864
command: cp /var/tmp/pveupload-651fa0b939061fb69fd14d1578e7006e /mnt/pve/SanBox/template/iso/ubuntu-14.04-server-amd64.iso
TASK ERROR: import failed: cp: cannot create regular file `/mnt/pve/SanBox/template/iso/ubuntu-14.04-server-amd64.iso': Operation not permitted


The server is set to store ISo's in this storage, just wont. Any suggestions?