Hi
I have made progress. It seems that the issue probably happens if you are booting in Legacy (non-UEFI) mode and have a USB drive >2.2TB connected.
I inititally tried swapping my SATA drive to a UEFI boot Windows system and booted it with the 3TB drives OK, so I have spent the morning converting my Debian system to use grub-efi and created the necessary partitions etc. It now UEFI boots into Debian with the 3TB USB drives connected. Success.
However it suggests that the Legacy BIOS is failing if it detects a >2.2TB USB drive even if you have told it not to access Removable Media and disabled USB devices in the list. I think that this must be a BIOS bug.
Peter