Currently Pair 2 (1 tB) is active and works OK.
Great. That that increases the likelihood that Pair 1 is also ok, and the box just refuses to mount it.
Your advice: " ... prepare a Linux PC to analyze the disks ..." can you please point me to some readings ? Live CD
, Low level file recovery
With some luck you don't need to recover anything, just mount the data partition. Connect one disk to your Linux box (by sata, e-sata, or an usb to sata converter).Find the device node of the data partition; open a terminal, and type
The size is given in kB, you are searching for an almost 750GB partition. I'll assume it's sdb2. Mount it:
sudo mount -t xfs -o ro /dev/sdb2 /tmp/mountpoint
If this works, you can use the GUI to access your files in /tmp/mountpoint. The partition is mounted read only, in case the filesystem was overwritten. In that case you should not write anything to the disk, to prevent further data damage.
Will the "apple" flavor of Linux do the job ?
What do you mean? Linux on an Apple? Afaik an Ubuntu Live CD will also boot on an (Intel) Apple. Or is there some distro called apple? Actually any Linux which supports the xfs file system (which is almost any Linux distro younger than, say, 4 years) can mount the disk that way. Even the 2Big can, if you have a shell. When the filesystem is overwritten it becomes a little more complex. Theoretically still each Linux system works, but not for all flavors all recovery tools are available. It is possible to compile PhotoRec for the 2Big, but that's a different story.