@pavlos thanks for the recommendation of ddrescue, that worked to recover my data to another hdd. I’m still a bit miffed I was able to copy 8TB of data to a hdd and then it becomes unallocated, not once but twice !!
Also noticed that when I format the hdd to ext4 file system in Openmediavault, it reserves a few MBs, when I use Disks or GParted far more space is reserved.
No I haven’t tried those, the command line is fairly simple, though you can make an error pretty easily when using disks of the same size.
I have since come across several packages / OS that rescue data. DDRescue seemed to work ok, albeit it took 33 hours for a 8TB drive.
What I hadn’t noticed in the past was that the overhead used for the file system can vary dramatically, from 168MB to 300GB on my 8TB drives.