Your environment includes a server that runs Windows Server 2008 R2 with the Hyper -V
role installed. The server hosts several virtual machines (VMs).
When you attempt to start a VM named VM1, you receive the following error message:
The application encountered an error while attempting to change the state of ‘VM1’. ‘VM1’
failed to start. Microsoft Emulated IDE Controller (Instance ID {83F8638B-8DCA-4152-
9EDA-2CA8B33039B4}): Failed to Power on with Error
‘The chain of virtual hard disks is broken. The system cannot locate the parent virtual hard
disk for the differencing disk.’
Failed to open virtual disk ‘V:\VM1\VM1.vhd’. A problem was encountered opening a virtual
disk in the chain of differencing disks, ‘IDE/ATAPI’:
‘The system cannot find the file specified.’ Failed to open virtual disk ‘V :\ VM1\VM1.vhd’. A
problem was encountered opening a virtual disk in the chain of differencing disks, ‘L :\ Base
\W2K8R2EERCBASE+.vhd’: ‘The system cannot find the file specified.’ You need to be able
to start VM1. What should you do?

A.
Merge the differencing disk.

B.
Compact the differencing disk.

C.
Reconnect the differencing disk.

D.
Convert the parent virtual hard disk.

Explanation:
Using differencing disks
A differencing disk is a virtual hard disk you use to isolate changes to a virtual hard disk or
the guest operating system by storing them in a separate file. A differencing disk is similar to
the Undo Disks feature because both offer a way to isolate changes in case you want to
reverse them.
However, Undo Disks is associated with a virtual machine and all disks assigned to it, while
a differencing disk is associated only with one disk.
In addition, Undo Disks is intended to be a shorter-term method of isolating changes.
A differencing disk is associated with another virtual hard disk that you select when you
create the differencing disk.
This means that the disk to which you want to associate the differencing disk must exist first.
This virtual hard disk is called the “parent” disk and the differencing disk is the “child” disk.
The parent disk can be any type of virtual hard disk.
The differencing disk stores all changes that would otherwise be made to the parent disk if
the differencing disk was not being used. The differencing disk provides an ongoing way to
save changes without altering the parent disk. You can use the differencing disk to store
changes indefinitely, as long as there is enough space on the physical disk where the
differencing disk is stored. The differencing disk expands dynamically as data is written to it
and can grow as large as the maximum size allocated for the parent disk when the parent
disk was created.

You can find out which parent disk a differencing disk is related to by inspecting the
differencing disk. Inspecting the parent disk does not identify any child disks that may be
associated with it. To make it easier to identify relationships between disks, we recommend
adopting a naming convention to support easy identification.
Managing the contents of differencing disks You can distribute the contents that are stored in
the differencing disk by merging the differencing disk. You can merge the changes to either
the parent disk or a new virtual hard disk. Merging to the parent disk modifies the parent disk
with all the changes stored in the differencing disk, and then deletes the differencing disk.
Merging to a new virtual hard disk retains both the parent disk and the differencing disk in
their current state, and creates a new virtual hard disk that is a combination of the parent
disk contents and the differencing disk contents. You use the Administration Website to
merge the changes. For instructions, seeMerge a virtual hard disk. Using multiple
differencing disks with one parent disk You can associate more than one differencing disk to
one parent, which means that virtual machines can share one parent disk but have their own
differencing disk. This way of using differencing disks can be useful in a variety of scenarios.
For example, a test engineer or call center technician could have a dozen or more virtual
machines with different configurations, such as allocated memory or type of networking, with
different software updates, and different installed applications. The virtual machines could
share a parent disk that contains the operating system, and each virtual machine could have
its own differencing disk to store the configuration that differs from the parent. The following
figure shows an example in which each differencing disk stores a different version of Internet
Explorer.

You can chain differencing disks, which means that a differencing disk can have another
differencing disk as a parent disk. Depending on how you design the chain, you can save
considerable disk space. For example, if you want to test upgrade scenarios or version
compatibility, you could use a parent disk as the base and a chain of differencing disks for
the consecutive versions. This approach would save disk space if each differencing disk
contained one update only. The following figure shows a chain of differencing disks being
used to store operating system updates.