How to see a GPSmap 278 as a mass storage device?

[smokefree]

I have a GPSmap 278, FW version 3.50, in fully working condition. However, when I hook it up to my PC (Windows 7, latest Garmin USB drivers installed), it is not recognised at all. It also does not power up when connecting to USB. Do I need to change any setting for it to be recognised as a mass storage device?


Edit 1:
When connecting the USB cable while the unit is on, the screen shows a message "USB connected", but still I am unable to see the device on my PC.

Edit 2:
Holding the "UP" button for 30 seconds when connecting the device to USB (which I read somewhere) does not help either: the device stays off. Tried a different (Garmin original) USB cable, still no luck.