Garmin Express: no support. Garmin Map Updater: refuse to install

First thing is to wait awhile and try again. Might actually be a Garmin server problem. 

It is also possible that any firewall you may have running is blocking the map updater. Easiest way to check is to turn it off temporarily and see if that fixes the problem. If so, you can go ahead and reconfigure the firewall to permit it on a permanent basis.

Next step would be a support call.

Not sure what's wrong. I just plugged in my 66i and used Garmin Express to update the TopoActive maps on the unit. (It's still chugging along, but it DID do it.) 

Garmin Express did inflict TWO updates on me - so I am now at 7.4.2.0. I am not sure of my 66i F/W version, but it should be up to date. Express did not offer a f/w update.

You might try removing the 66i from your Express installation and then adding it back.

Eventually did these things to get Garmin Express working, not sure which one or the combination is important:

- Use mobile app to update 66i itself from 6.x to 8.x version

- Switch 66i's internal system settings, interface to MTP mode

6.x is a long time ago. Still, I would not expect this to block Express access. Which mobile app? (Assuing GCM)

The other possibility is that you were not in mass storage mode. MTP vs. one of the other modes should not matter as long as the device ended up in "mass storage mode". Garmin Spanner will ask whether or not to go into mass storage mode. Garmin Serial goes directly to mass storage mode. NMEA In/Out also goes to mass storage directly, as does Text Out. MTP mode is a different protocol for mass storage (Media Transfer Protocol vs. USB "disk").

Bottom line is that the only way to avoid mass storage mode is to be in Garmin Spanner mode and decline to enter mass storage mode when you plug the unit in. That would block Express access.