Share external Storage

I use OwnCloud on my Mac. There is an USB Harddisk i add this Disk wit external Storage to the own cloud
like /Volumen/OwnCloudDisk as local storage.
Now i want the share one directory for one user on this disk.
But i cant share any Directory on the usb disk with the admin page of onwcoud
the share symbol is missing (only on this usb drive)

Also new in 9.0 is an option for the ownCloud admin to enable or disable sharing
on individual external mountpoints (see Mount Options). Sharing on such mountpoints is
disabled by default.