My MiniCore/MicroCore does not seem to boot up properly after a reboot? All I can see is a "spinning" line on the video overlay?

The reason is that the fail safe mode has accidently been entered by sending serial data to the unit too soon after reboot.

To ensure that the camera can be recovered after a failed firmware update, a special fail-safe mode exists. In the fail-safe mode, the camera will stay in the boot-loader for 10 seconds waiting for a serial transmission. If a serial port transmission is detected during these 10 seconds the boot-loader will enter service-mode where the Cirrus software can be used to download new firmware to the unit. If no serial transmission is detected  during 10 seconds the boot-loader will launch the existing firmware.

To prevent accidentally entering this mode, communication during the first 10 seconds after camera power-on should be avoided. An alternative solution is to use the command "GCT" after power-on, in order to probe if it is the fail-safe bootloader mode or if it is the firmware application that is running:
If "GTC," returns "GCT,1,B,1" the bootloader is executing. If "GCT,1,A,X" is returned, the firmware application is running. To exit boot-loader mode and launch the firmware application, send the command "EXE".