Can't boot SS2 from external scsi disk

[duepmeie at idtsun4.kfk.de]

We just got a diskless SS2 from SUN and  I want to connect to it a
external shoebox with a WREN VI 94241-502 SCSI disk and a 150 MB tape
drive. 

I jumpered the disk as SCSI ID 3, connected the shoebox to the SS2 and run
the installation procedure for 4.1.1 Rev B.  The formatting of the disk
and installation of the OS worked as it should be, but when I rebooted the
machine after completion of the installation, I get right after the reset
the error message

Can't read disk label
Can't find Sun disk label package

I thought something was wrong with the label of the disk, so I booted the
the machine from the network and run format, but format showed, that
everything should be ok with the disk. So I assumed, that the boot strap
code was wrong and installed a new boot block with installboot and made a
second bootstrap on the h partition of the disk.

When I rebooted the machine I got exactly the same error message, but when
I booted the machine from the h partition with

boot /sbus/esp/sd at 3,0:h -a

I could boot the machine without any problemes, so that it works with sd0a
as root, sd0b as swap, etc.

Can anyone explain me, why I cannot boot the machine from the sd0a
partition as it should be??