The aim of the operation was to enlarge the space in the database volume. My colleague replaced the disks in the storage (hot plug). I was forced to perform the entire operation remotely.
The LSI controller saw the new disks, however the OS and the Iscsi software thought that the device was still 360GB in size. I configured the controller and created a new 700GB array. I had to force the OS to change its mind…

I used the following option:

 
echo 1 > /sys/block/sdc/device/rescan

After that FDISK saw the proper size.
And from now, the b/m command shows that the backend-store (LUN1) disapeared

tgtadm --lld iscsi --op new

So i added it, again:

tgtadm --lld iscsi --op new --mode logicalunit --tid 2 --lun 1 -b /dev/sdc

Voila!