Yes, I formatted the Nuvi in the mistaken belief it would help a pre-existing condition where the unit was booting randomly, IE sometimes it worked, sometimes it bootlooped at map loading. The unit is my dad's, but previously I'd had a 660T and currently I have a TomTom GO Live 950, both of these I'd formatted in the past with no ensuing problems and this had helped with their random reboot issues.

The lesson I've learned here is that checking for hidden folders when backing up the contents is crucial (Thanks lolypop000 for rescuing me from that one) and formatting using a non-windows PC is also fraught with danger (Thanks dasilvarsa for the pointer to rmprepusb post)

Through the use of this forum, its members and gpspassion I finally managed to undo my bad choices at the start.

As with all of this sort of mucking about with proprietary gear, it is a case of YMMV (your mileage may vary) and beware the leopard.

Thanks once more to all who helped.