Then for sure the previous detail map file was corrupt and therefore causing the device to shut down when it got to "Loading Maps" in the boot cycle. By enabling MSM which GarminExpress uses exclusively (many earlier softwares such as WebUpdater and MapInstall use Garmin Mode) you've been able to overwrite the faulty map. Sometimes there may be a small corruption in the data inside a map IMG file which doesn't cause a problem immediately but the device will be 'soft-bricked' by that at some later time.

And yes, perseverance is key to getting a modern device to flash using Updater. You need to be very fast but once the flash is initiated the device will hold preboot. Miss the window of opportunity by a fraction of a second and you have to start again. I had one stubborn bricked device which took dozens of tries to flash.