@julianof
I initially mis-read your post. In (2) i do see that you flashed cure fw but Disk Management hasn't automatically assigned a drive letter. To fix that simply right-click the drive information and select "Change Drive Letters and Paths" to assign a drive letter manually. If still having problems proceed as below:
I'm unsure why you attempted the tests described, other than (1) if it was cure fw. The complete guide for both making and flashing cure fw is detailed in the initial post here: [Only registered and activated users can see links. ]. Incidentally, the only part of fw which is modified by Cure3.exe is fw_all.bin. The boot.bin and resources.bin are unchanged.

Regarding your attempt to load cure fw via the SD method (4), that is usually impossible because a soft-bricked device likely cannot move far enough into the boot process to see a media card. In a healthy device you will note that "Loading Maps" appears on the device screen prior to a flash being initiated from the card. Consequently, any corrupt file which must be loaded (even if that is one of the map IMG files, loaded last) will have 'bricked' the device before the point of recognizing the card. Regardless if you still have an SD card inserted you should take it out because it might contain a bad file.

Behind the spoiler is a cure kit which should work for you:
Spoiler: Click
[Only registered and activated users can see links. ]

Instructions:
  • Extract the ZIP file somewhere convenient to find the folder "Flash_Kit"
  • Open the folder to find "110601000630.rgn" and "Updater.exe"
  • Drag the RGN onto Updater which will open it's GUI
  • Enable the Radio Button next to "USB Device" on Updater's GUI and leave it open and visible on your desktop with your mouse cursor positioned over the OK button
  • Ensure your nuvi's battery has at least some reasonable charge in it, don't ever try flashing when it's flat
  • Ensure the device is completely OFF - press and hold the power button down between 10 and 30 seconds if necessary
  • Connect a 'known-good' USB cable directly (no hub) to your Windows computer, use only a rear USB2 port of a desktop. Avoid a front port or USB3 port if possible
  • Holding the nuvi in your left hand with fingers behind it, use your THUMB to hold the upper LH corner of the screen firmly, maintaining a firm and steady pressure
  • Use your right hand to plug the cable into the nuvi's USB port while watching the window next to 'USB Device' on Updater's GUI
  • IMMEDIATELY you see the device details populated in the window adjacent to "USB Device", click OK or just hit the Enter key of your PC's keyboard
  • Keep firm steady pressure on the screen until Updater shows a success message or at least hold it until "Software Loading" or similar message appears on the nuvi's screen.

Once you have cure fw loaded and Mass Storage Mode is re-enabled you can proceed as detailed in the Cure3 thread to remove corrupt files. If you need to re-format the file system do so only using [Only registered and activated users can see links. ] with the recommended settings. Note that re-formatting should be a method of last resort if you're absolutely unable to determine which file is bricking it (do a backup anyway beforehand), or if it has severe file system corruption, or it's been mistakenly previously formatted as NTFS rather than FAT32.