Symptoms:

Upgrading virtual hardware in ESX 4 may cause Windows 2008 disks to go offline. 
Upgrading virtual hardware from version 4 to version 7 may cause Windows 2008 disks to go offline.
Creating a new virtual machine on hardware version 7 may cause Windows 2008 disks to go offline.
Note: The first disk (C: drive) is not affected. Any additional disks may be affected and show marked as Offline within Disk Management.
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VMware vCenter Update Manager is a component of VMware Infrastructure that automates patches and upgrades of ESX hosts, virtual machine Tools and hardware, Windows and Linux virtual machines, and virtual appliance. A new white paper, VMware vCenter Update Manager Performance and Best Practices, is now available.

In this paper we discuss VMware vCenter Update Manager 4.0 host deployment, latency, resource consumption, guest OS tuning, high-latency networks, and the impact of on-access virus scanning. We also provide performance tips to help customers tune the system for better performance.