Are you sure system does not try to boot from new disks?
I'm pretty sure I've cycled through all four disks in the BIOS boot order,
although it's possible. It's somewhat difficult because all four disks are
identical and the BIOS doesn't provide a way to distinguish one from the
other.
I've also even tried to rearrange the disks physically by rearranging the
disk trays in the chassis...
I've also installed grub on all four disks. Wouldn't this serve to direct
the boot information to the disks with /boot and the real root?
Well, GRUB prints dots when it loads parts of core.img from disk.
Single dot would imply single extent which is OK. Then it jumps to it.
Now if for whatever reasons it got garbage from disk. of course
attempt to execute it will be unsuccessful.
Could you boot any live media, run
https://github.com/arvidjaar/bootinfoscript/raw/master/bootinfoscript
and send result to me?