Please advise if you manually created a folder named Garmin, because it may not be capable of booting empty. Also whether you then added other folders/files if needed following what i suggested in Post #21 should it still not boot:
Quote Originally Posted by Butters View Post
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Remove everything and then add back files/folders progressively starting with empty Garmin folder before loading original fw (it may refuse to boot if there's nothing at all present) ..........
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Finally the third way, is to reformat with RMPrepUSB using only the recommended settings which is what i'd do only if the using one of above methods become too onerous and removing the most likely files hasn't allowed it to boot.

If it still won't boot with just an empty Garmin folder then add back progressively the least-suspect, i.e. Voice, Text etc. folders with their files. However many devices will simply boot completely empty and self-recreate the Garmin folder and its GarminDevice.xml so try that first.
Alternatively, are you still using the PC which initially only recognized the device as a removable device? Best use another computer if that's the case as there may be other settings/restrictions on it causing the weird behavior of RMPrepUSB.

There's another (statistically less likely) explanation which is my earlier concern, i.e. of the device having a physical flash problem. It's odd that you were able to format using the Windows built-in tool but not initially with RMPrepUSB using appropriate settings. Usually that's reversed in my experience. Perhaps if there is a flash failure it's intermittent at present but that'll be progressive and it might simply fail completely while being stressed and time will tell with constant flashing. Hopefully it won't die.