Edit: It was not 4TB disk but 6TB disk in fact. (We actually ordered 4TB disk...)
I think the problem of the backup disk was the flaky power supply for the external drive.
I swapped the drive to a new HGST 4TB one, but it was neither recognized nor spun up with the external power supply we had. So I decided to put both the new and old drives in the PC chassis to power them up with the internal power supply. I tested the old disk via a USB-SATA cable. However, this disk was not recognized. I noticed that the disk was not HGST 4TB but Seagate 3TB. Is it possible? I thought it was 4TB... Did I miss something?
Once the new 4TB was connected to the USB-SATA, it was very smooth to get it mounted. Now the disk is mounted as /media/40mBackup as before. /etc/fstab was also modified with the new UUID. All the command logs are found here below.
Let's see how the morning backup goes. It would take a while to copy everything on the new disk. So it was actually very nice to set this disk up by Friday midnight.
controls@chiara|~> sudo mkfs -t ext4 /dev/sdd1
controls@chiara|~> sudo emacs -nw /etc/fstab
controls@chiara|~> cat /etc/fstab
controls@chiara|~> sudo mount -a