Backing up Hyper-V Server?

I've got a Hyper-V Server 2008 (i.e. Core) box running the following:

1. A Windows 2008 domain controller and fileserver.
2. A backup domain controller (also Windows 2008).
3. An Ubuntu server web server.

I'd like to ensure that this is backed up to an external (USB) hard disk, but I'm running into some problems.

1. I can't mount the USB hard disk in any of the guests (or can I?). This means that the backup has to be done from the host.
2. The file server guest has several physical disks mounted (i.e. they're not VHD files). This means that I can't back them up from the host.

How should I go about backing this up?

Update

I'm going to try the following:

1. Plug the external disk into the host, mount it and share it.
2. On the Windows PCs, use WBADMIN to do a complete backup to the network share (e.g. \\HOST\USBDISK).
3. I'm still wondering about how to back up the Ubuntu box to the external disk. I'll raise another question about this.
4. I'm also still wondering about how to go about scripting/scheduling this -- I can't use the built-in Windows Backup schedule, because the external disk might not be available.

I've managed to (temporarily) get hold of another Hyper-V-capable PC, so I'll try a disaster recovery scenario at the weekend.

Firstly, there is a massive gotcha with Hyper-v and passthrough disks. You can not back them up normally through any kind of Hyper-V style back. The backup just ignores these disks and backs up the VHD.

For VHD backups, I'm currently trialling a method of taking VSS file system snapshots and then using these to pull consistent backups of the VHD files. Works pretty well for Windows machines due to the integration services. We do an export of the VM before it goes into the backup set so we can have a copy of the configs for importing. I'm also working on a way to only store VHD diffs to reduce backup storage requirements for 7 day rolling backups but it has yet to work properly.

With VSS, it will pause any machines that don't respond to the VSS aware query. Your ubuntu machine is one that'll be affected and probably have a few seconds of an outage during the backup while the snapshot happens.

Best bet is to use the new Server 2008 Windows Backup. It integrates with VSS, will quiesce running machines to get a consistent backup and then back them up to the drive you specify. Works pretty dang well and once you get it setup to automatically run the backups you just have to check on it every so often. Much better than the old NTBackup stuff.