I know you're a bit worried and somewhat confused about how to proceed with this and i'm also guessing that the nuvi is important for you to have working properly... Please rest easy, i'll "hold your hand" so to speak and we'll work thru this together. If you don't understand anything below, just ask me first, otherwise below is the 'step-by-step' guide with files likely to fix it:

Extract [Only registered and activated users can see links. ] somewhere convenient to find 3 folders (QuickCure3; Cure3_Firmware-HWID1371; and Original_Firmware-HWID1371).

Do this in turn strictly as required:
  • Open QuickCure3 folder to find 137101000830.rgn and Updater.exe
  • Drag and drop the RGN file onto Updater.exe, that will open its Graphic User Interface (GUI)
  • On Updater's GUI, select the Radio Button next to "USB Device", leave the GUI open and visible on your PC's desktop and have your mouse cursor positioned ready over the "OK" Button (don't click it yet)
  • Proceed as before to put your nuvi into preboot and as it connects IMMEDIATELY click the OK button or you can just hit the 'Enter' key on your PC's keyboard (watch the window on the GUI next to "USB Device", when it populates with the device's details that's when it's connected)
  • Keep holding firm pressure on the nuvi's screen until you at least see "Loader"/"Software Loading" or similar message. If unsure, just hold the screen until it finishes (you'll know it's finished because the nuvi will only start to reboot after it's been completely flashed)
  • The nuvi should now only boot as far as the GARMIN logo screen, don't be concerned - that's expected and desirable. If that's what it does, leave it connected to the PC and it should be detected in MSM after a short time, so now proceed with next step to reformat with RMPrepUSB. NOTE! - if it's not detected automatically in Windows File Explorer and RMPrepUSB, check in Disk Management if it's shown there without having been given a drive letter - if it doesn't show at all in Disk Management then remove it from PC and reconnect to check again but if still not detected at all there then look below (*)
  • Now use RMPrepUSB, only with the settings recommended in the thread to re-format its file system. Then safely disconnect it from the PC
  • Open the folder "Original_Firmware-HWID1371" to find another RGN and Updater, drag and drop as detailed before
  • Put it back into preboot as previously described and flash the original firmware.

*Only if QuckCure3 doesn't re-enable MSM, do this (but there's no need to do it otherwise):
Open the folder "Cure3_Firmware-HWID1371" and flash it in pre-boot as previously described, then use RMPrepUSB and finally flash original fw as detailed above.

If neither QuickCure nor Cure3 work to re-enable Mass Storage Mode, the device clearly has a different problem. Good luck.

PS: We've still got big weather problems here so my response to any reply might be delayed.