I believe that is what I have tried. This method has always worked in the past.

I start with Garmin Express and am told there is an update available. With Zummo hooked up to my Mac via USB cable, I follow the instructions to install update. The installation begins running but always stops at 17.3 ...

Garmin won't discuss this with me as they do not support the 660 any longer.
I cannot update. When the process starts, it consistently freezes at 17.3%. I have a 32G Sandisk card in the unit which has worked for years.

Any help appreciated.

BTW I am 77 yr old non tech guy so please reply in simple ...