The symptom: update-grub (as run after the installation of a new kernel package) fails because it's 'unable to find [a] GRUB drive for /dev/sda2 - check your device.map'. This happens using both grub-legacy and the new grub-pc package.
You'll probably have run into this if your guests were created using xen-tools.
- The way Xen pokes /dev/sda1 and /dev/sda2 (which are probably LVMs on your dom0) into your guests without a corresponding block device confuses grub
- grub used to be dumb enough to ignore the mysterious presence of 'partitions' with no block device and carry on anyway
- The versions in Debian Squeeze are just clever enough to be dangerous and fall over in this situation
Happily, we've worked out the fix. Grub is hard-coded to do the right thing in this situation if your disk devices are called '/dev/xvd[a-z]', so you need to:
- Fix all references to '/dev/sda2' to read '/dev/xvda' (and /dev/sda1 to /dev/xvdb). In practice this means grub's menu.lst and possibly your /etc/fstab if that isn't by UUID
- Edit the /etc/xen/guest-name file on the dom0 to rename the partitions (change the 'root' specifier and the lines in the 'disk' list)
- Destroy (sudo xm destroy) your guest. Note that you must destroy and recreate it; rebooting won't give Xen a chance to rename the devices.
- Bring it up again with xm create -c config-file