Geoffrey! If you don't sit up straight and start paying attention i'm going to ring your Mum, so listen up:

Read this post and the couple preceding it before you do anything further: [Only registered and activated users can see links. ]. In your case, the RGN file should be named 194801000781.rgn because you reported you have V7.80 firmware loaded. So do the SW Version override as explained in that post #180, but to 781. Read the notes at base of this post and if all confirmed now follow this:

1. Ensure you've removed gupdate.gcd from the unit's .System folder. Note that gupdate.gcd may not be present but must be removed if it is.

2. Drag 194801000781.rgn onto Updater.exe and enable the radio buitton to the left of 'USB Device' in Updater's GUI. Leave it visible for the rest of this procedure.

3. Plug a suitable USB cable into a REAR port of your PC.

4. Hold the upper LH corner of the device screen firmly and plug the other end of the usb cable into the nuvi which will power it on. Watch the GUI closely.

5. Immediately you see the device's details appear in the window to the right of 'USB Device' press 'OK' on Updater's GUI or just hit Enter key on your keyboard.

6. Keep holding the screen at least until 'Software Loading' or similar message appears, or hold until the whole process finishes to be sure.

7. Disconnect nuvi from PC and let it boot normally.

Notes:
Things to be absolutely sure of first. Look at your 'Inventory' page to double-check HWID and SW Version. [Only registered and activated users can see links. ]. Top line should read 'Boot BLK B1948-00 780' [HWID; Firmware version].

The reason to increase the SW Version by .01 is in case we might need to 'fool' the unit to accept the apparently same Version, i.e. As Jotne found with his 2699 it may refuse to reflash another V8.20 file but happily accepted V8.21.