Instinct 3 update 8.09 stuck at "Restarting..."

Then I would contact the dealer and have the watch exchanged. If you bought it directly from Garmin, call the Support again, and insist on replacing the watch. Your case is not typical, although a lot of people had problems with the update, in most cases (including myself) it was relatively easy to fix even without deleting the hidden folder. You seem to have had a bad luck with an exceptionally bad exemplar. Getting a replacement should help.

Did you turn off the watch - hold the "GPS" button down until it mounted in windows? 

If you start it without GPS button being held down during the whole process, the ExtData-folder will not be visible.

Have you kept the GPS button pressed while watch was turned off while connecting to computer ? I didn't see the ExtData folder if I had it on or didn't keep the button pressed

The computer doesn't recognize the watch while I'm holding the GPS button - and after a while it reconnects in the regular mode, where the folders are not available.

I guess I can try yet another computer

Well, for me it made the sound like a new device was connected and then the drive showed up in the drives list, only then I released the button. Weird it's not working for you

You can also try repeating it several times. My Instinct 3 also initially did not show the hidden folder ExtData when using the method with the GPS button. I thought it was due to the fact that my watch was already updated to 8.09, but I tried it a few hours later again, and now it shows up.

For me it didn't worked either from 5.#, I deleted only the update without any outcome then did a full reset of the watch and I was able to update it by keeping the GPS button pressed while connecting to the PC after the full reset, once the update appeared on the watch I request to apply it, seems like is freezing in resetting mode but after a min a progress appears

Same Issue here !