Startup disk info is stored in NVRAM (PRAM). You didn't mention resetting it. In case you don't know how, read How to Reset NVRAM on your Mac.

Another point: Can you boot successfully from the SD backup?


Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365