Additional info: rereading the mobo manual, I realized I had plugged the HDs into SATA 3 and 4. When the C: drive is connected to SATA 1 and to SATA 2, the machine doesn't even get as far as the BIOS.

When I try to access the boot menu, I get a message saying, "The system cannot find any bootable devices." The machine runs (or at least, ran) Windows 10, without any difficulty. The GPU is the only ...