GarminExpress doesn't detect devices in preboot/Garmin mode. It's limited to use in Mass Storage Mode or Media Transport Protocol ONLY and cannot access devices in preboot, like for example Updater.exe which is used to load firmware only as an RGN file in preboot.

Certainly immediately AFTER you successfully loaded the fw made with Cure3, the device would have re-booted with Mass Storage Mode and Media Transport Protocol re-enabled. The MSM/MTP should not have disappeared again after you loaded the original fw. Try completely closing GE (check that it's not running in the background) and rebooting the PC, then if GE still doesn't see the device post again with more details.