Microsoft’s Small Business Server 2008 is supported running as a Hyper-V guest; but there’s one nasty problem. The built-in backup expects external USB drives, and a Hyper-V guest does not have direct access to USB.
Here’s a solution I’ve come up with. It lets you use the built-in backup wizard, and lets users simply attach a new external USB drive each day as they expect. It is not perfect, since it requires copying the entire backup afresh to the USB drive, rather than doing a differential backup – though SBS itself still does a differential backup. It also requires Hyper-V 2008 R2, which means struggling with server core if you use the free version. Still, it’s better than any solution I’ve seen from Microsoft.