I paid the USD$30 ransom for a hassle-free RMA with 2-way shipping included. The replacement Seagate 8TB drive arrived today via UPS, and has been inserted into the system's backup array (currently re-syncing 4TB of data to it).

All good for now.

This escapade forced me to finally deal with the 99% full backup array, by replacing one of its 4TB drives with the RMA replacement 8TB drive. Now the backup array is the same size (20TB) as the main array. Yay!