Hearing a chime is a good sign. When the device is in preboot it will not show in Disk Management however it should definitely be shown as connected in Device Manager under "GARMIN Device" when in preboot. If so, it's a bit of a mystery why it shows there in preboot but not under Disk Management in Mass Storage Mode as usually if the device is healthy it'll show properly both ways so please check again in Device Manager when you're certain it's in preboot.

This still may be a software problem. So a question: What are you trying to flash with Updater.exe when you get that message "No updates ...."?