Dear Edward,
I have the same problem. My 6125 was connected via a CA-53 USB cable that seems to be OK because I have been able to back up my phone contents with it. Bluetooth was off and Nokia PC Suite was closed.
The software update process initiates all right, the current firmware version is 3.71 and the update 3.72, but after a while the software update program shows a message saying "Connection to phone lost. Your phone could not be detected. Please reconnect the USB cable. Click 'Retry' to continue with software update."
When I click "Retry" the program retries but ends up showing the same message again.
At this point, my phone screen shows "Test Mode". It won't turn off, so I remove the battery and turn it on again.
A *#0000# check shows that the software hasn't been updated.
Please let me know if you have any further suggestion.
Raul Andrade