After the addition of a hard disc to a mirrored zfs rpool, the boot process immediately stopped with the GRUB message: "bad pbr sig". It means that something is wrong with the Primary Boot Record (aka Master Boot Record). The solution was simple - I swapped the boot order of the hard discs in BIOS. Now it works perfectly again.
NEW: Online Workhop Effective WebApps without Frameworks is also coming to: MUC Airport.