Well, i don't think it's bricked anyway. You have the reverse of the obvious symptom for a 'bricked' unit. A bricked unit cannot be seen in Mass Storage Mode. If i'm right about GVN5x not having a preboot mode then it cannot be flashed by an RGN file using Updater.exe because Updater.exe only works in preboot.
If its file system is empty that's probably the reason why it won't boot, many Garmin devices cannot boot empty they require at least some files in them.
Connect it in MSM then create a folder named Garmin in the root and it might boot up.
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