Each of those WD Blue 5,400rpm 6TB drives draws peak 1.75A each according to the specifications.
I think you have two choices: find HDDs that draw less peak amps
-or-
Uprate/upgrade the power supply to something that supplies more than approx 7A. The general rule of thumb used for the figure was 3.5A for the RockPro64 + 2x 1.75A for the HDDs = 7A+.
Beware, I am unable to be certain that will even work.
I think you have two choices: find HDDs that draw less peak amps
-or-
Uprate/upgrade the power supply to something that supplies more than approx 7A. The general rule of thumb used for the figure was 3.5A for the RockPro64 + 2x 1.75A for the HDDs = 7A+.
Beware, I am unable to be certain that will even work.