Got out my old Nuvi 1300 with an 8 gig micro SD, intending to update the maps. But when I connect it up to my Windows 10 Pro x64 desktop nothing shows up in Explorer. It just shows as USB Mass Storage in the Safely Remove etc menu.

In Disk Management it shows as 1.86 gig unallocated. And there's another disk (I assume it's the SD) which Disk Management insists must be initialized. Garmin Express cannot see it either.

But there's nothing wrong with it! It works just fine with the old maps that are on it.

I don't want to create new partitions where it needs none and end up bricking it.

Have you tried another cable / USB port or another pc? It sounds like a big mistake when all works fine and windows detects two storage devices as unallocated space. You can also try to access the Nuvi from a Linux Live System.