G'day kazumoto, welcome to Noeman. Konichiwa.
If PC can see the zumo's internal files, try to remove the file GUPDATE.GCD first. It is probably that file which was loaded when you were trying to update firmware which is causing the problem.
If it is not present or removing it does not allow the zumo to boot, then backup the internal files and reformat with [Only registered and activated users can see links. ], then connect to WebUpdater or Garmin Express.
If you wish to access pre-boot in zumo 660 to update the firmware using rgn file with updater.exe [safer imo], from memory the procedure is this:
* Have a USB lead ready, plugged into a rear port of your PC.
* Ensure the unit is fully off and remove the battery cover.
* Hold finger on lower left corner of screen and keep constant pressure.
* Connect the USB cable and hold the power button while the zumo boots up [keep holding finger on the screen].
* Release power button and quickly press OK on updater.exe window to flash fw. [have the rgn already loaded with updater.exe visible on your desktop].