In previous releases when the disk image of a virtual machine was being modified or used by a task the entire virtual machine was locked. This lock is now more specific and applies to the disk. This means that users are still able to perform additional tasks on the virtual machine, such as adding or removing disks, concurrently.
As a result of this change of behavior the virtual machine status may indicate that a virtual machine is "Up" when the attached disk is in fact being used by another task. Users will not be aware of this lock until such time as they attempt to perform an action that requires a lock on the disk.