• Powering on the virtual machine fails.
• You see the error:
  
  Reason: Thin/TBZ disks cannot be opened in multiwriter mode..
  Cannot open the disk '/vmfs/volumes/4c549ecd-66066010-e610-002354a2261b/Windows Server 2008/Windows Server 2008.vmdk' or one of the snapshot disks it depends on.
  VMware ESX cannot open the virtual disk, "/vmfs/volumes/4c549ecd-66066010-e610-002354a2261b/Windows Server 2008/Windows Server 2008.vmdk" for clustering. Please verify that the virtual disk was created using the 'thick' option.
   
• Cannot open Thin/TBZ disks in a multiwriter mode.
• VMware ESX cannot open the virtual disk.

This is an expected behavior, where the format of a virtual disk in multiwriter mode is changed to an unsupported format. For more information about the requirements of virtual disks in multiwriter mode, see Enabling or disabling simultaneous write protection provided by VMFS using the multi-writer flag (1034165).

To confirm the issue, verify the virtual disk format. To check the disk format, see Determining if a VMDK is zeroedthick or eagerzeroedthick (1011170).

To prevent this issue from re-occurring, ensure that virtual disks in multiwriter configurations are not converted to the lazyzeroedthick format. To use disks in multiwriter mode, the format must be Thick Provision Eager Zeroed.

To work around this issue if you need to extend a virtual disk in a multiwriter configuration, extend the virtual disk by running the vmkfstools -X command from the ESXi/ESX host command line (this is vSphere Client default behavior). This ensures that the virtual disk format stays in the correct eagerzeroedthick format.

For example, to extend the size of the virtual disk to 60 GB:

# vmkfstools -X 60G -d eagerzeroedthick /vmfs/volumes/Datastore_name/virtual_Machine_folder/virtualmachinedisk.vmdk

Notes:

• In the preceding vmkfstools -X example, 60 GB is the size that you want the disk file to be, is not the size by which you want to extend the disk file.
• VMware recommends that you perform this operation outside of production, as it writes to the disk and takes significant time to complete.
• If the disk is already extended, you can use Storage vMotion to change the disk format. For more information, see the Migrate a Virtual Machine to New Storage section in the vCenter Server and Host Management guide.
• For virtual machines that are running Microsoft Cluster Service (MSCS), ensure the disks containing the guest Operating System is not connected to a SCSI controller that is engaged in bus-sharing.