When going to vMotion a server either manually, via DRS or after putting the ESX host into Maintenance Mode you may receive the following error message and the migration stalls:

“Unable to Migrate from <ESX Host target name> to <ESX Host destination name>.  The operation is not allowed in the current state.”

“Unable to migrate from <ESX Host target name>.  Virtual machine is configured to use a device that prevents migration: Device ‘SCSI Controller 0’ is a a SCSI controller engaged in bus-sharing”.

Screenshot below:

The reason for this error message is that your SCSI controller is not set to share the attached virtual disks.  By entering the Virtual Machines properties and viewing the offending SCSI Controllers SCSI Bus Sharing section you will likely notice that it is set to ‘Physical’. 

To resolve this issue either set the ‘SCSI Bus Sharing’ type to either  ‘None’ or ‘Virtual’.  Your VM will need to be powered down before you can make a change.

Whether these disks need to be shared with other VMs will determine which one of these two settings you select, ie:

None – the virtual disks won’t be shared with other VMs.

Virtual – the virtual disks can be shared with other VMs on the same ESX host.