First thank you for all the help in this forum.
After confirmation that download:
MBL - Firmware Version 02.43.10 - 048 (6/22/2015)
MBLD - Firmware Version 02.43.10 - 048 (6/22/2015)
have been indeed valid firmware versions for MBL and MBLD I could move on to figure out why the hell the updates are not working on my book army.
What I can tell search for updates clearly not working any more.
So I went on to try it with the downloaded firmware versions after a few factory resets and downloads I finally got NAS-01, NAS-02-DUO and NAS-03-DUO updates to version 02.40.06-048 but with the MAIL I ran out of luck.
So I went another way.
1. on NAS-01: create disk image of 3TB MBL
#: dd if=/dev/sda of=/DataVolume/shares/Public/mybook3tb_nas-01.img bs=1M count=4420
2. on NAS-01: copy image to usb-stick or in my case copy it to the usbstick already mounted on the recovery system.
#: scp /DataVolume/shares/Public/mybook3tb_nas-01.img root@192.168.20.103:/mnt/usb/mbl
3. on Recovery: flash disk image to new HDD in my case the
250GB seagate for MAIL
200GB seagate for TORRENT (thinking of aquiring even a fifth book ;-))
#: dd if=/mnt/usb/mbl/mybook3tb_nas-01.img of=/dev/sdb bs=1M
4. on Recovery: unplug the drive and put it to a windows system
(parted and gparted giving error: partition outside disk! and could not find out how to get it fixed via linux)
Under Windows use System->Drive Explorer to view the new HDD, ignore the ridicolous large size info 3TB even for the 200GB drive.
5. on Windows: delete false data partition.
6. on Windows: unplug the drive and put it back into the Recovery System
7. on Recovery:
#: parted /dev/sdb
(parted) mkpart primary 4624M -1M
(parted) quit
#: sync
#: mkfs.ext4 -b 65536 -m 0 /dev/sdb4
8. on Recovery: unplug the drive and put it back into the MBL enclosure.
Connect ethernet-cable and power it on.
after a few minutes the LED should turn green
9. on MBL UI: restore to factory settings
10. on MBL UI: reboot
11. on MBL UI: activate ssh
12. on MBL SSH: ssh in (root welc0me)
mkswap /dev/sda1
DONE!
After that I got a working MBL even with smaller HDD than 1TB, but this steps should work with any drive size I’ll purcase in the future. 
