A key aspect of managing storage for your Hyper-V hosts is backing up the virtual machines that run on these hosts. Backups can be performed in various ways depending on the type of storage you’re using and the tools you have available.
An important distinction is whether the backup of a virtual machine is performed online or offline. Online backups involve no downtime of the virtual machine, while offline backups involve some downtime.
For more information, see this post from the Virtualization Blog: http://blogs.technet.com/b/virtualization/archive/2008/08/29/backing-up-hyper-v-virtual-machines.aspx
Of course, even backups can have problems sometimes, and in this section Thomas Roettinger describes how to troubleshoot ...