Canada & Mexico missing after maps update

How did you copy the maps to the computer's local drive?

Check what maps Basecamp can see by doing the following:

• Connect your device to the computer
• Make sure Garmin Express is not running
• Start BaseCamp. 
• Give BaseCamp time to access and read the data from the device.  You will see it listed in a "Devices" category in the upper left pane.
• In BaseCamp, go to the Maps pulldown menu.  It will list all the maps BaseCamp can access, both on the device and installed on the computer.  Same version of a map in two places will be noted with its location by either the device name or "Installed" if on the PC.

This is what mine looks like.  Notice the first entry is the previous map that I have not deleted from my PC yet.  Also notice that a map appears to be listed twice on the connected zumo XT.  This is because the map file is too large to exist as one file on the XT so is broken into two files but when it is installed on the PC it is installed as one file.  Looking at this list won't solve your problem but it may provide clues into what is going on.  BTW, BaseCamp comes with a Global Map by default.  This map appears to be useful, but only to look at. It can not be used for navigation.

In Garmin Express I went nuvi 50 > Map Details > Map Options > Install to Computer. Then I installed Base Camp.

Here are the maps I see in Base Camp. "Garmin (G:)" is the SD card that is installed in the Nuvi, so it looks to me like the 2026.1 maps are installed on the computer and on the Nuvi, split over the device and the SD card as you described.

So I think the maps are installed on the Nuvi, but it can't find them or doesn't recognise them.

In base camp, if I choose Maps > ( installed ) or Maps > ( garmin ) I can see detail in Canada, for example this screenshot from Stanley Park in Vancouver:

If I choose Maps > ( nuvi 50 ) then the detail disappears. Apparently I can't include a second screenshot, but all the streets, green spaces and bridges disappear. It's the same thing I see on the device itself, like it's not accessing the SD card where the map detail is.

Sounds like a faulty device or sd card. Try a master reset of the device and see if that resolves it.

Did you format the SD card as FAT32?  A new card would likely come pre-formatted as exFAT.  Perhaps the fact the you can see map detail in Basecamp is because a PC would be fine with reading an exFAT formatted card but the Nuvi requires FAT32 formatting.

It's a new 32 GB SD card and I created a GPT partition table and 25 GB partition, which I formatted FAT32. Maybe the nuvi doesn't like GPT or the small offset of the first partition. I will try MBR and simpler layouts if necessary.