I think I missed something simple.

[gdb]

Thanks for the suggestions! I had written the same image to two different SD cards with the Pine64 Installer, and it did appear to write correctly (no errors on write), but neither image booted. I chose a different image and wrote it to one of the SD cards and it booted correctly. With no video output, I mistakenly assumed that the Rock was dead. I am glad I was wrong! Thanks again.