Help needed fixing a broken Windows 2008R2 SQL cluster

We had 2 Windows 2008R2 servers using Windows Clustering Services to create a HA SQL Server instance. They are using iSCSI shared storage. One of the servers failed to rejoin the cluster after a reboot and has been shut down. The failed server appeared to have been removed from the cluster correctly. The remaining server was working fine until it too was reboot and now the cluster won't start. 

The Cluster Management recognizes there is still a cluster and that the working server is a member node. The Cluster Service can be started but the cluster itself never initializes. Using Force Cluster Start fails.

I get errors like this:

Node 'SERVERNAME' failed to form a cluster. This was because the witness was not accessible. Please ensure that the witness resource is online and available.

One problem I can see is that the shared iSCSI volumes (Quorum, Data etc.) are mounted on the remaining server but marked as Reserved, Offline, Read-only and Clustered. I assume that to resolve the issue I need to get them online on the remaining working member of the cluster but can't figure out how. Here is the Diskpart output (Disk 0 is the boot drive, Disk 1-4 are the clustered drives):

DISKPART> list disk

  Disk ###  Status         Size     Free     Dyn  Gpt
  --------  -------------  -------  -------  ---  ---
  Disk 0    Online           50 GB  1024 KB
* Disk 1    Reserved        200 GB  1024 KB
  Disk 2    Reserved       1545 MB  1984 KB
  Disk 3    Reserved        100 GB  1024 KB
  Disk 4    Reserved       1545 MB  1984 KB
DISKPART> select Disk 1

Disk 1 is now the selected disk.

DISKPART> attribute disk
Current Read-only State : Yes
Read-only  : Yes
Boot Disk  : No
Pagefile Disk  : No
Hibernation File Disk  : No
Crashdump Disk  : No
Clustered Disk  : Yes

DISKPART> attribute disk clear Readonly

DiskPart failed to clear disk attributes.

DISKPART> attribute disk clear Clustered

The arguments specified for this command are not valid.
For more information on the command type: HELP ATTRIBUTES DISK

DISKPART> attribute disk clear ClusteredDisk

The arguments specified for this command are not valid.
For more information on the command type: HELP ATTRIBUTES DISK

As you can see from the output I can't clear the Read-only or the Clustered Disk state. How do I resolve this?

Thanks,

Daniel.