Be sure you are using the most updated version of Express. Once Express is updated do any firmware updates for the Zumo. One way to fool Express into thinking that the SD card is your device is to clone the card. Part of the problem you're having is your Zumo uses a hidden .system folder and Media Transfer Protocol. You need to change the GPS to be a USB mass storage device. The issues are described here:
https://support.garmin.com/support/sw/supportPage/display?locale=en_US&topicName=zumo 665
To use this procedure on these devices, you must first
set your computer to expose the hidden \.system folder. If your device operates in MTP mode (Media Transport Protocol),
change it from MTP to Mass Storage mode before you continue.
Note: Most of the newer devices operate in MTP mode by default. Turn on the Zumo and go to the Volume setting screen. Hold the upper right corner until a menu appears. Select MTP mode and change it to USB Storage. This lets you access the GPS folders like you are used to, and allows you to make a backup and see the map files.
To clone a SD card, insert a clean SD card in your computer. It should be formatted with FAT32. Create folder \Garmin on the SD card. Copy the file garmindevice.xml from \Garmin folder of your device into the \Garmin folder of the SD card. The garmindevice.xml file now causes Express to identify the card as the Garmin device
Leave the SD card in the computer's card reader and safely remove/eject/unplug your Garmin device from the computer.
Install and run Garmin Express or Garmin MapUpdater. You can download
GarminMapUpdater for PC with this link. It should still work.
When Express or MapUpdater is finished, you should have 3 map files in the Garmin folder of the SD card: gmapprom.img, gmapprom.gma and gmapprom.unl. Rename these files to gmapsupp.img, gmapsupp.gma and gmapsupp.unl.
Garmin Express should work in MTP mode, but it's nice to make a real backup, and a lot of us want to see the files. The SD cloning trick should work for you and avoid the need to even plug in the GPS. Files transfer to the SD faster than to a GPS.