In-Place Upgrade Only for Virtual Machines with RDM Disks
Details
How can I upgrade an ESX Server host that has virtual machines
with RDM disks from ESX Server 2.x to ESX Server 3.0?
Solution
Currently, you cannot migrate powered-on and powered-off virtual
machines with raw device maps (RDMs). This means upgrade procedures
that use migration to reduce virtual machine downtime are not
possible when virtual machines with RDMs are involved. Only an
upgrade without migration is possible.