Downloading map update to/installing on Mac without device plugged in?

That's weird, I always update to my PC without my devices plugged in. Maybe the Mac version works differently?

Do you have a backup of your device?  If so you can create a clone simply by copying the Garmin folder containing the file GarminDevice.xml to a SD card or usb stick, that will fool Express into thinking your device is connected.

It seems that connecting the device is required for a number of maps. I contacted Garmin "Support" regarding my desire to update City Navigator North America NT to ver 2020.30 in BaseCamp Mac without my (lost) nüvi 255W, and they basically said that I was dumb out of luck. In my case, it's a Proof of Purchase of the map that came with the device. Lo and behold, I found the device after nearly two years of absence, and all is well.

Garmin apps on the Mac are not easily 'fooled' with connected SD cards or usb sticks. Perhaps if the file was on a virtual device created by JaVaWa Device Manager or similar, yes. Just realize that using JaVaWa to create that virtual workaround is not possible anymore in MacOS Catalina. 

I've not heard of anyone with a Mac struggling to use a cloned device, did you actually try it?  Unfortunately JaVaWa isn't updating his programs anymore so Mac users with the recent OS are out of luck as far as his programs go, but you don't need his programs to create a virtual drive, just follow his instructions.

I did try it, long ago, but I do recall having no success. I'll make sure I have device backups and keep your suggestion in mind if I ever run into that again.

So, it looks here http://www.javawa.nl/virtualdevice.html that Disk Utility is another route for creating a virtual device. Good to know.