Obrigada pelas dicas.
Afinal foi mais complicado... O GPS nem era detectado pelos computadores. Estes foram os passos que tivemos de percorrer para ter sucesso. Fica aqui a informação, pode ser útil a outros.
Obrigada ao PauloMatos, também geocacher, pelo trabalho

e pela descrição de todos os passos .
A. eTrex 20 firmware image files can be downladed:
http://garmin-etrex.wikispaces.com/firmwareconnect via USB, transfer file to device, disconnect device, turn off and then back on, and then back-on, and wait!!! device will reset when it's done.
B. If you have the message "System Software Missing"
1. Connect gps via usb
2. Unpack ZIP file (attached: update_etrex20.zip, you will have to rename UPDATER.ex_ to UPDATER.exe) or available at:
http://www.mediafire.com/?579do7s4fwd676ahttp://www.mediafire.com/download/579do7s4fwd676a/updater_etrex20.zip3. After decompressing, it will show 2 files: one ****.rgn and other UPDATER.exe
4. Drag file ****.rgn up to UPDATER.exe
5. A window will open where you should select the way gps is connected. (USB port).
6. Then OK and it will start erasing and installing software into device.
7. Very important:
--- Don't disconnect GPS from USB port, it will take several long minutes (with no apparent activity)
--- On your computer screen it will appear a message stating that transfer was completed (click ok), window will go away BUT your GPS still has the same message on screen, wait for some minutes without doing anything (be patient!) until it ends. DON'T DISCONNECT YOUR GPS!
--- It's only safe to unplug, when you see the USB logo on GPS screen and your computer recognizes your GARMIN..
If by any chance device is connected and not recognized on step 5, install USB drivers (see link bellow).
There are other references that might be useful for use with this device.
USB Drivers software version 2.3.1
http://www8.garmin.com/support/download_details.jsp?id=592Plug-in Garmin Communicator
http://software.garmin.com/pt-PT/gcp.htmlUpdates & Downloads
http://www8.garmin.com/support/download_details.jsp?id=5553WebUpdater (Windows XP SP3 and newer) software version 2.5.6
http://www8.garmin.com/support/download_details.jsp?id=931