In order for a storage repository to be available to a server, a connection must exist between the server and the SR. This connection is provided in software by a Physical Block Device (PBD) . A PBD stores information that allows a given SR to be mapped to a server. A PBD must be attached or plugged in to the server in order for the SR to be available. If a PBD is unplugged for some reason, the SR will no longer be available to the server and will appear with a broken storage icon in the Resources pane.
You may be able to diagnose and resolve some common SR connection problems using the Repair Storage Repository tool. In the Resources pane, select the storage resource, right-click and click Repair Storage Repository on the shortcut menu.
Alternatively, on the Storage menu, click Repair Storage Repository.
The available storage repositories are listed, and you can see their status.
Click Repair to have XenCenter attempt to repair the storage. Progress and results are displayed within the Repair Storage Repository dialog box.