VMware has released a KB with a handy vmfstools command to check your VM’s disk status.  If the VMFS output shows a “Z” it is Zerothick with blocks that have not been written to yet. Read the article here. This allows you to check the disk status for options such as Fault Tolerance or Microsoft [...]

VMware just posted an alert on this for ESXi 4 Hosts managed by either vCenter 4 or 4 Update 1.  If you remove and re-add the Host to vCenter, or upgrade vCenter to Update 1 you may experience this issue. At this time, there is no official fix, however it appears a reboot should fix [...]