Well, Iīve try to re-flash the backup with the SD with the following combinations:
1: SD:\Garmin\Updater\1715\ldr.bin (2797)
2: SD:\Garmin\Updater\1603\ldr.bin (2797)
3: SD:\Garmin\Updater\1715\ldr.bin (3795)
4: SD:\Garmin\Updater\1603\ldr.bin (3795)
Same result, the nuvi stay bricked. Apparently the point 3 in the Neil's scenario itīs true, the card never was checked. Could confirm this, if there were SD with a light that turn on when accessed, like the old pendrives.

Neil, catymag, kunix, thank you very much for your help and suggestions!