There is an error in the original firmware of the GPSmart that the device almost never get a fix since July, 11th 2005. The function to calculate the visible satellites is erroneus and gives wrong values since that date. So the device searches for satellites which are not visible.
On this page on eXpansys, johnr_61 writes that he patched the firmware with new ephemeris data so that everything works fine again. This patch should work until 2010-11-22.
Connect the device with the special cable. (Normal Mini-USB cables don't work since the devices actually provides only a standard serial interface
which has to be converted to normal USB using the
PL2303-IC from Prolific.)
Use FortunaSync to establish the connection (Link to Fortuna GPS) and then use SirfFlash to upload the new firmware.
You might have noticed that you can't power the GPSmart using a normal USB-to-MiniUSB cable. The backlight for the display can be switched on and off, but the display itself doesn't show anything. Since I had the same problem with my HTC Universal, I found a solution at forum.xda-developers.com. According to Wikipedia, the 4th pin is used for detection of USB On-The-Go, i.e. whether the other device provides power or not. Interestingly, the roles seem a bit mixed up here. According to Wikipedia, if pin #4 is grounded, the socket should provide power. Here it is that pin #4 has to be grounded for the device to accept power (for charging).
After opening the GPSmart and bridging pins 4 and 5, I could successfully power the device via a normal USB-to-MiniUSB cable. After removing the bridge, the device powered off.
My first idea for a solution was to use conductive lacquer coat to bridge the pins inside the MiniUSB plug. But then I remembered I had some adaptor:
And lucky for me, the pins were already bridged in this thing. So I can now power my GPSmart using a normal USB extension cable and this adaptor.
The MiniUSB port on the GPSmart is not really a USB interface. The GPSmart has a serial interface and also uses the port for power. The FortunaSync cable uses a FTDI FT232BM which also controls pin #4 (see above). This IC converts the serial signals to USB.