Hi,
Try this,
1) Right-click on the desktop disk image and select Get Info.
2) Sharing and Permissions will appear at the bottom of the resulting dialog.
3) Before you can change them, you will likely need to click the little padlock icon below the list of permissions to unlock them.
4) Enter your password when prompted.
5) Click the Read only entry next to everyone, and select Read and Write, then close the dialog box.