I think I found a defect pertaining to the USB driver. I do not know if it is specific to asuswrt or asuswrt-merlin, because from day one, I loaded Merlin's firmware.
What happens is that on boot, with the HDD connected, the OS reports that it had detected sda or sdb, but it does not detect the partitions, sda1, sda2, ... or sdb1, sdb2, ... Thus, the drive's partitions cannot be auto mounted.
After this happens, if I remove the drive from the USB port and reattach it, the devices are detected correctly and are auto mounted.
What happens is that on boot, with the HDD connected, the OS reports that it had detected sda or sdb, but it does not detect the partitions, sda1, sda2, ... or sdb1, sdb2, ... Thus, the drive's partitions cannot be auto mounted.
After this happens, if I remove the drive from the USB port and reattach it, the devices are detected correctly and are auto mounted.