The device 'extracts' (or executes if you like) a GUPDATE.GCD from the (normally hidden) .System folder of the internal memory. In some outdoor devices the file is then automatically deleted but not in automotive devices.

It is possible to start GUPDATE.GCD from a Garmin root folder of the card. In a heathy device it will be read at the end of the normal boot cycle, and if the device determines the firmware in the GCD is a later version than is presently loaded it will flash it from there. The process is the very much same as if the GCD is in the internal memory which also requires that the device can boot. Note the use of 'heathy' and 'normal'.

It's also possible to write individual firmware BIN files from the card directly to the relevant regions of the flash memory but it's not possible to initiate (i.e. run) firmware directly from the card to start the device as you ask. Certainly some map IMG files and some other files such as points of interest GPI files can be used from the card. Even the initiating of BIN file to flash them from the card requires the device to be able to boot almost fully (usually just after the point in the boot cycle when maps would be loaded) to even examine the card. That process requires the system software (aka firmware) in region 14 to initiate enough to in turn initiate from SD the boot.bin renamed as ldr.bin (i.e. the ramloader). It's also possible to directly initiate flashing of firmware BINs from the SD in preboot using a modified boot.bin as an RGN file.

Regardless, i don't think such processes are helpful in your case because you can access preboot normally and appears to successfully flash the original fw that way although it then will boot-loop still. It flashes the Cure fw to allow MSM however but Cure fw doesn't fully boot the device, it intentionally stalls the boot-cycle before essential operating files are loaded. Therefore if it still boot-loops when empty or when 'known-good' folders and files are on-board the problem's elsewhere and most likely of a hardware nature. You can read about flashing from SD here: [Only registered and activated users can see links. ]. Info about modding ldr.bin in [Only registered and activated users can see links. ]. Try it if you want but if it doesn't work then a hardware fault is confirmed absolutely, sorry.