I was doing some rewiring and didn’t realize my Core and the attached devices were powered up. Somehow, I managed to short the D1 pin to 12V and it remained that way for a few minutes before I found my Spark Core hot and completely nonfunctional.
I discovered by using a multimeter that GND, 3v3 and D1 are shorted together. I tried removing the 3v3 regulator first (because it was easy), but that was not the culprit.
I’m going to try disconnecting the D1 micro pin from the board to see if that makes a difference.
Originally, I was posting asking for advice as I hadn’t figured out which pin to cut. I dug into the STM32F103CB data sheet and figured out that D1 = PB6 = Pin 58. I’m firing up my microscope and trying to precisely cut the right pin. I decided to leave this post up since it might benefit someone else with the same issue.