OK, i think i understand now. I misunderstood before because you had the card 'E' in the 'Safely Remove Hardware and Eject Media' pop-up showing in your first screenshot in Post #11. I assumed you'd done that intentionally. My mistake. However, I'm unsure why you have drawn a question mark in your latest screenshot for Win 7 Disk Management. If you are thinking that the unit should be displayed there in some way, that's not the case with these older nav units. The modern nuvi-type units do show there when connected in Mass Storage Mode, but 26x0 SP era units do not show nor do they have a drive letter allocated.

So, in precis - In Win 7 on another PC you can see the unit in 'Devices and Printers' correctly and it's recognised by BaseCamp with no problems. But in Win 8.1 you get the yellow triangle error and it's not recognised by BC.

I guess a picture is worth a thousand words and i realise that English is not your first language. In 8.1 Devices and Printers, and then C:/Windows, please follow this:

BC 8 1 2610

You should have both gmngen.sys and gmnusb.sys shown under 'Driver File Details'. If either or both are not shown then reload the current Garmin USB driver version 3.2.1.0. If both files are shown, then proceed to C:/Windows/System32/drivers. Locate the 2 files, and check that the required permissions are allowed for 'All Application Packages' for both of those files.