Thanks for making a response.
I have re-read my post. What I have written is not correct (too late at night here in China to write clearly). What I should have said was:
I have flashed the firmware to level L. This was no problem.
I obtained the provided software upgrade via the BENQ website.
When I tried to upgrade the software from 6.7 to 7.2 the following happened:
1) The upgrade unistalled the exisiting software (version 6.7)
2) The upgrade then failed to install the new software.
3) I got a message saying that the upgarde could not find a suitable drive.
According to Device Manager, my drive is an ATAPI DVD DUAL 8x4x12.