A teammate ran a test and confirmed that the patch Juerg attached solves the problem - on an MBR disk over 2TB, it limits the automatic growth to 2TB rather than destroying the partition table.
Thomas, if you agree that this issue is Severity: important and believe that the release team would also agree, would you please upload this quite minimal fix to unstable? I can file a corresponding bug in the BTS tomorrow, after which you can make an unblock request. The release policy allows this severity of fix to migrate to Jessie by Dec 5 for optional or extra packages, so if nothing goes wrong we are not too late.