More than 50% of our virtual host installations are failing constantly on the backup when the guest machines are in a running state. What was once a nuisance is now prolific. In every case simply shutting down the guest machines allows the backups to complete successfully. The guest range from SBS2003 to 2003 terminal servers to 2003 or 2008 DC's. There is no common thread or application loaded on any of them that explains the failures. If Microsoft is serious about Hyper-V then a RELIABLE backup solution needs to be rolled out asap. Having your entire backup fail because a single non-critical guest machine cannot backup while online is a very bad design. If anything at all is wrong on the guest machine then the backup of the entire server will fail for that evening. This is a chronic problem and frankly I'm not sure what to do. We now have dozens of Hyper-V host machines in the field that will never backup reliably on their own. I've got to find a solution and so far about a hundred collective manhours on this has yielded virtually no positive results. When we do get one working then it will run for a week or so and then start failing again. Hyper-V and Windows Server Backup needs constant attention. It shouldn't be that way. Contacting Microsoft has been useless so far. I'm wide open to any advice anyone can offer. Surely there must be a way to make it work reliably.