Am already in touch with email support about this. But just to let you know - I did try to change the boot option and reboot from the USB stick, but when we get to the menu with
- 32bit system (2.6.27)
- 64bit system (2.6.27)
- Run software update installer
and choose Run software update installer, the system again says "no units found -- continue -- exit" (and only exit can be pressed).

Open-e phone support seemed to think that the problem could be that our initial installed capacity was 16 TB (for the onboard system which is at v 3278) whereas in the 64 bit trial we added 48 TB worth of new disks. So they asked us to apply our 32 TB additional license key to version 3278 itself before attempting the upgrade. We tried that as well (i.e .rebooting from the on-board module rather than the USB stick). For some reason it says key invalid so I am trying to resolve that with email support now. Will let you know here how that goes.