Garmin Drive Assist 51 Lmt-D sd card problem

Most Garmin satnavs only have one microSD card slot, but your Garmin Drive Assist 51 has two microSD card slots, one for holding files like maps etc. and another for recording dashcam video, the maximum sizes for these two drives are not the same, the memory card (Item 5) can't be bigger than 32GB whereas the video recording card (Item 3) can be 64GB, so in a nutshell your 64GB card will only work in slot marked 3

 

You don't say what the update was for, it is very unlikely that a map update would cause this problem, I guess it is more likely to by caused by an operating system update but I think it would effect every user and there would be a lot more users complaining, Garmin are not keen on users going back to older versions of maps or operating systems so they don't provide any way to do this.

I would try copying all data on the 64GB microSD card to your computer and then re-formatting the card in the computer, that may work, if not try a different microSD it can be smaller than 64GB if you have one