I performed a network upgrade (with boot disk via ftp) from 6.0 to 6.1. The install script died while attempting to find the partition where 6.0 was installed. The debug info indicated that it had difficulty reading /dev/hda5. I ran fdisk. /dev/hda5 was an extended partition that was marked as ext2 but not formatted. Formatting the partition and re-running the install solved the problem.
This issue has been assigned to a developer for further action.
If you were trying to upgrade a machine with a partition which was marked as Linux Native, but has no valid filesystem on it, bad things are sure to happen. Will mark as a request for future development that we handle this case more robustly.
Fixed in 6.2.