Bootlooping devices can be tricky to flash however the flash chip may have physical damage as already suggested in Post #125. When you get the message from Updater.exe that it's "unable to program the requested memory region" it means that it's not able to write data to a specific region which it's trying to flash, i.e. in your case Cure3 firmware contains the boot.bin intended to be flashed to region 12 (a virtual region), fw_all.bin to region 14 and resources.bin to region 127 (both physical regions).

If there's actual damage (think, like 'bad blocks') to the flash memory, which involves any of the relevant permanent regions, then the flash fails with that Updater message. You can keep persisting in trying to flash it and hope to fluke it but the fact that you've seen it remain in Device Manager for a while means that it's holding preboot (not dropping it in a few seconds like some bootlooping 'soft-bricked' devices do).

You can also try flashing it from an sd card using a modified boot.bin as RGN file in preboot with Updater.exe. It's less than simple to make the required kit for that so i've made one for your nuvi but if you use it the risk is all yours.

Preparation: Please use only a Windows PC, preferably a desktop and connect only to a REAR USB2 port. Avoid using a front USB, hub or USB3 port. Have a suitable 'known-good' miniUSB cable with the A plug inserted ready in the selected port of the PC.

Now look behind the spoiler below:
Spoiler: Click to view

Download this ZIP file and extract it somewhere convenient to find a folder named 'Garmin' and two files, 097201000630.rgn and Updater.exe:
[Only registered and activated users can see links. ]
Proceed as follows:
  • Prepare a CLEAN microSD card of no less than 8GB in size formatted in FAT32 only, re-format using Windows if necessary.
  • Place the folder Garmin onto the card.
  • Drag 097201000630.rgn onto Updater.exe to open it's GUI and select the radio button next to 'USB Device', leave the GUI open and visible on your Desktop with the mouse cursor positioned over the 'OK' button.
  • Ensure the nuvi is completely OFF. Because it's bootlooping you might need to force it off by holding the power button down.
  • Insert the sd card into the nuvi.
  • Hold the screen in top LH corner and insert the mini-B USB end of the cable into the nuvi (provided you have the other end properly plugged in to the PC already, the nuvi will start to power on).
  • Watch Updater's GUI and the instant you see the device detail appear in the window to the right of 'USB Device' click 'OK' on the GUI or just hit the Enter key of the PC's keyboard.
  • Keep holding the screen until you see "Software Loading" or similar message on it, if not sure just keep firm pressure until the flash either finishes or fails indicated by a message from Updater.
  • If it's successful and there's been no physical failure of the memory regions the device will reboot but it'll stick on the Garmin logo screen (as designed) however with normal PC access as Mass Storage Mode should be re-enabled.

Report back results. Hopefully you'll now have Mass Storage Mode available. We can go further only if it has indeed worked as intended.

Remember, this is solely at your risk if you choose to use the above recovery method. I've taken all care in the making of the SD kit however if your device has a failing flash memory the chip may just die at any time and each flashing attempt puts additional strain on it.