Mac Pro 2013 that struggles to power a third USB 3.0 self-powered hard drive
Isn’t the Mac Pro supposed to power up to 4 USB 3.0 drives?


It depends.

The specification for USB 3.0 is 5V and 900mA per bus.   A spinning hard disk has a power requirement of about 1.8W (5V at approx. 350mA)*.   That's just for the drive not counting the enclosure power requirements.  4 drives would mean  1400mA or 1.4A which is well above the available power supplied.

If you want to drive 4 USB drives, you want to get a USB hub with a power supply capable of supplying enough power (i.e. a USB hub with a 3A power supply)



* Specifications for a WD Black 2.5" 1TB HDD internal drive; USB drive wasn't used due to lack of power requirements noted.