I have a first version NanoPi M4 with a SATA hat, but I have never run any drives from the USB. Is there any way you could externally power those 2 HHD’s and see if it’s a power issue?
BTW: This is an excellent SBC machine. It does become a little pricey when you add the heat sync, 4-port SATA hat, and dedicated 12V. power supply. It is my backup server and has been running continuously for 2 years.