Update ...
Looks like the update to ESXi 5.5 failed in a very ugly way.
The ESXi 4.1 was installed to a 1Tb disk - with about the first 420 Gb used.
Sometime later the existing VMFS-volume in primary partition 3 was expanded with the help of an extent. This extend used the free space left on disk - about 500 Gb.
Now my task is to recreate a partitiontable that enables me to present a partition so that ESXi again considers it to be the missing extent.
What a nightmare 
I love nightmares 